Dear Fallen ~ Although I do not take Lamictal myself, I will share what I know of my 15 year old daughter being on it. As far as whether it will cause weight gain, you are in luck there, because of all the meds out there, Lamictal is highly chosen (as I did so for my daughter) because of it being one of the very few that are weght neutral.

Of course, everyone will have their own reactions to meds and I am sure that there have been some that will claim that they gained weight with Lamictal but from my daughter's experience she has not.
Okay....breast tenderness and menstrual cycle...my daughter has not complained of any breast tenderness. And she is on the pill so her cycle is regular. She was on the pill before starting the Lamictal but I have heard that if you add the pill afterwards it will require more Lamictal to reach the same stability as somebody who is not on the pill. So you may wish to keep that in mind.
The rash...oh was that something that I was scared of for my daughter, particularly since she has devleoped rashes and sensitivities to 4 other meds!! And so we went even slower on the increases than usual knowing that she is sensitive to many meds. It took us 5 or 6 months to get to the 150mgs that we are on now which is alot longer than the usual. That is because of my insistance with the pdoc because the way that I saw it, Lamictal was a med that I had pretty much hand picked for Erin because of it's wonderful side effect profile!!! And I didn't want anything to deny us the ability to stay on it, so if going slower meant being able to stay on it without getting the rash we were willing to do that.
I heard the Steven Johnson Syndrome is quite rare in adults and more of a problem in adolescents/children. Erin developed some swollen lymph nodes which is what usually precedes the rash and when she did we went a step back on the med for a week and then back up on it just to be on the safe side. She did develop a rash and we stopped the med and had her evaluated by a dermatologist who determined that it was not the Steven Johnson type rash. And so we resumed the Lamictal and have been doing well ever since. One thing I have heard is that you can develop the rash at any time during an increase but once on the right dose it is very rare.
I was also told that the rash to look for will be patches the size of a silver dollar coin and filled with fluid appering blister like which will burst and slough off like burned skin. When and if a rash occurs you should not take another dose until evaluated. So even if you do get a rash it may be something else so do not get too alarmed. Just remember, that if it is Steven Johnson the most important thing is not to feed it anymore Lamictal or it will become severe.....if looked into promptly it won't be bad. Most of all remember that it is very rare and the only real side effect that one has to worry about.
Another thing that I wanted to share is that women in particular will see an increase in their bipolarity during that time of the month. Our pdoc did as I have read and that was allowed for an increase in the med during the week or Erin's period. In Erin's case he upped the Seroquel but I have heard that once you are at the right dose you can do the same with the Lamictal if you should run into that problem.
Oh....and as far as each time you up the dose expect a few days or the first week to have some increased anxiety and/or agitation. It will eventually level off during the weeks that follow and then go up a little before you are due for your next increase. You will KNOW when you reach the right dose when you have no further dips when you are due for the usual increase. Lamictal works well sometimes all on it's own but at times it needs another med to balance things out. As I have shared here many a time it works well with those who are more on the depressed side of the bipolarity.
I hope this helps and feel free to ask any questions that you may have.
Oh and if you are sensitive to meds you may want to ask your pdoc if you could start at 12.5mgs the first week rather than the 25mgs....they say the slower that you introduce the med the less of a chance you will have of developing the rash.
Good luck and know that I am here if you should need any more reassurance.
((((HUGS)))) ~ Goody