Hey twinkies...
Are you using a condom when the two of you are intimate? If you are not, and your boyfriend is ejaculating inside of you, then it could be the blood left over/dry blood on the inside walls that are now mixed with a liquid, and then the blood is discharged.
If this is not the case, I would definately check with your doctor to make sure everything is ok, and to find out a solution to the problem