Frequent painful cramps in 2nd trimester?

I'm 15 weeks 5 days pregnant with baby #2. I got a few cramps early on and my doc said it was my uterus stretching. They stopped for awhile, but now i get them when i go up and down the stairs, lift something with difficulty, or do much physical work at all. My son is 24 lbs, and i get cramps when i pick him up. Sometimes i get cramps for no reason at all. They're almost debilitating, and i have to sit still and groan until they go away. There is no bleeding, but i still don't feel that it's normal. I have my next pre-natal appointment tomorrow, so i will ask then. But in the meantime, has anyone else had this? Is this something to worry about? I missed my vitamins for awhile, but have started taking them again. Help? Thank you.