By "regular milk" do you mean breast milk or cow milk? Breast milk is best and is generally recommended for
at least the first six months, but the longer you can do it, the better.
Cow's milk can cause some problems for human babies since it wasn't made for them; it was made for cow babies, which have different nutritional needs. Many babies are allergic to a protein in cow's milk. They often outgrow the allergy by the time they're two or three years old, but that could be the cause of the issues you're seeing now. Cow's milk can also cause iron anemia in babies.
Formula isn't any better. Babies raised on a formula diet are far more prone to health problems than breastfed babies are. Breast milk contains antibodies; formula does not. Those antibodies help your baby fight off infections and other problems. Babies raised on formula are far more likely to develop pneumonia, MS, diabetes, weight problems, leukemia, heart disease, and many other issues.
Talking to a doctor about it sounds like a good idea.
Here's a collection of articles on the subject if you're interested in reading more:
http://www.vegsource.com/talk/parenting/breastfeed.htm