Hi,
I'm 39 and had this problem for a while, went to the doctor. I think you should be seen because there can be many reasons for this including more serious things. In my case, the problem was minor - I had a large number of capillaries exposed in my vagina, near my cervix, and sex was causing them to bleed. The treatment was a procedure called LEEP, which luckily healed the problem. But at the same time, the doctor had to rule out cancer, polyps, fibroids, etc. and did a hysteroscopy and biopsy. So... could be "nothing" but worth checking out.
As you get older, you don't produce the same amount of natural lubrication like you did when you were younger. You also might not be engaging in enough foreplay to allow for a more relaxed and naturally lubricated vagina. If your boyfriend is penetrating you a bit too roughly as well, this could contribute to tiny tears in your vaginal canal that can cause slight bleeding or spotting. Even the friction from a different position can contribute to slight bleeding or tearing combined with not enough lubrication.
Take more time for foreplay or buy water based lubricant from the drugstore to help things along more smoothly.
If its accompanied by severe abdominal pain or fever, you might want to see your doctor about a possibly more serious underlying condition.