This happened to me today and they did a blood test to make sure I didn't miscarriage and said no sex til they figure out what is going on.
It's certainly not uncommon to bleed after sex. Our cervix is so sensitive during pregnancy, and very easily irritated. A lot of women will experience some light spotting. In most pregnancies, sex is perfectly fine, and if this is post sex spotting, it's on the outside of the cervix and will not harm the baby.
It the spotting lightens up and stops, no need to be seen right away. Do mention it to your Dr. at your next appointment. If it gets heavier, or becomes outright bleeding call your Dr. tomorrow.
I'm sure it's fine. All the best to you!