What causes OCD and compulsions?

I have compulsions to buy more than one thing when shopping. Or I will return something and run back and buy it back cause it bothers me. I am not sure why I have OCD? I have 3 kids, 2 of whom are autistic. I am very overloaded and no help except from my husband. My mother is useless. I haven't had a day off in 10 years because I have no one to take my kids and give me relief. The only relief I get is now school that they are in elementary school. The thing is my OCD seems to be getting worse. I'll be leaving the house with my husband and have to go back and check the door several times. OR I'll check my kids at night and double check and triple check and more their window locks to make sure they are locked. I also, check the stove a million times before bed. Does anyone know what is the main cause of OCD. Also, when I go shopping if I see a sale, I have to buy the item (not just one handbag), but everyone in every color. Why is that. I guess I need that full feeling. Any suggestions what kind of OCD this is or why? Any advice is appreciated.