my son is four months old. I've only had my period once so far. I'm also breastfeeding. I'm late on my period but the…

my son is four months old. I've only had my period once so far. I'm also breastfeeding. I'm late on my period but the test was negative. had sex last night twice and now on my left side of my belly is really tender and hurts. the only thing I can think of is a cyst. I've had them a lot on my left side before. but has anyone experience this?