Spotting at 15 weeks

So I spotted two days after having sex when I was 14 weeks, it was a bit red then turned to brown, it was not much and did not fill any pads. I freaked out and went to the ER they did blood work and ultrasound and told me all is good. Yesterday I stared spotting again at 15 weeks and it just happen that it was a day after having sex. No pain just tender lower abdomen. Dr. Told me not to worry but I can't help it. Didn't spot on any pads just when I wipe. Anyone has a clue