Re: I have trouble digesting food and sometimes vomit it hours later undigested
The hives and rashes are, of course, indicative of an allergic reaction. If it's food related then you should keep a food journal and try and determine which foods are the most problematic and avoid.
If your gallbladder is acting up, it can cause some of those symptoms but most often it is not considered the root cause unless you are experiencing the typical GB pain. If you think this is a possibility, you might want to try following a gallbladder diet (which would exclude eggs, bad fats, and a few other items).
If this has been going on for some time (months+) another possibility could be that you have a slow emptying stomach (slow motility), which can be caused by a number of items, many are still unknown. The medical term often associated with this is Gastroparesis. There is a test (stomach emptying study) as well as varying treatment options for it.
Consider searching here and reading up on these items, even if they don't fit your problem exactly, a lot of suggestions on how to manage may be helpful to you.