Light cramping after sex?

I’m almost 22 weeks and after sex I have light cramping in my lower stomach. The last time I had sex, I was spotting pink about 2 days after.. it wasn’t heavy though and it was only when I would wipe or id notice a little on my panty liner. The ob said it’s normal but I’m still kind of scared because it’s my first pregnancy. I also have some lower back pain but I have had that all through out my pregnancy.