Hi, my story is the very same as yours. I have a 10 yr old daughter & when I was pregnant with her I didn't start to show until 5 mts. When I got pregnant again (my baby daughter was born 2 weeks ago) I was already starting to wear maternity clothes during my 3rd month. I was shocked
The doctor said all pregnancys are different and not to worry at all. I thought by showing so early I would be like a horse at 9 months. I gained the exact same weight with both pregnancys, and at 9 months looked pretty much the same as I did with my first. So in answer to your question, yes, you do start to show sooner with your second. My doctors explanation was that your body stores the memory of your first pregnancy and when it detects your pregnant again, it automatically starts to slacken the muscles in your belly in preparation for whats to come, plus retain fluids etc.