Side effects of Calcium supplements?

Thenoseknows is exactly right about the concern about unabsorbed Calcium... but to actually answer your question, it sounds like it was an allergic reaction. Your doctor apparently doesn't know enough about nutrition or allergies to address this, but yes, people CAN be allergic to minerals and even nutrients that have an RDA... I'm one of them (I'm allergic to Zinc) and I know others who are allergic to basic nutrients (Vitamin C and Vitamin E in different people). There are two possibilities for what's happening... you're allergic to the source of the Calcium (the Lactate can come from multiple sources, but can easily be Calcium reacted with Lactic Acid) or you're allergic to the Calcium itself. It's very rare for someone to be allergic to Calcium itself, but it's possible none the less. I would suggest trying a different type of Calcium to eliminate that possibility. Also, make sure you don't take more than 600 mg of Calcium (combined from all sources) at any given point in time as your body can't absorb more than that at once, make sure you take at least 1,000 mg of Calcium throughout the day, and you should ideally take it with some things that help the absorption of Calcium as well (such as Vitamin D, Vitamin K, Magnesium, Boron, Silica, Zinc, etc). Once you eliminate possibilities of what you're allergic to, you can go from there. When I found out I was allergic to Zinc, I had to try 3 different bottles until I found out it was the actual Zinc itself instead of what it was bonded to... it sucks, but it's the best way to find out for sure short of expensive allergy tests. Good luck and I hope I helped!