Bleeding during sex can be caused by things other than a tearing/eroding hymen (which is that whole "bleeding during your first time" thing that people talk about). It's usually caused by not having enough lubrication.
Your genital/vaginal tissues are really very delicate--think of the inside of your mouth. And when there isn't enough lubrication for a penis or finger or object to get in, the friction can result in irritation or little tears to that tissue.
Luckily, there's an easy way to prevent that in future: using condom-safe lubrication with your condoms or latex gloves. You can find lubes near condoms in grocery or drugstores, with brands like Astroglide or KY Jelly. This can help make sexual activity feel better for everyone involved. An added bonus is that they help to keep condoms from breaking!
If the bleeding continues or is very heavy, yes, you should get to a doctor, but if you're only having light spotting, keep the area clean and don't have sexual contact again till it heals up.
"And when the answer that you want/Is in the question that you state/Come what may..." - Coheed and Cambria, "Blood Red Summer"