Nursing and unprotected sex?

So I’m experiencing some cramps on my right side right now. It also feels a little like implantation pains. It’s concerning because we did have unprotected sex and used the pull out method since our daughter was born 5 months ago. I did take tests and neither pregnancy nor ovulation tests were positive. I don’t believe I could get pregnant because I have always needed fertility treatment in order to do so. Is what I’m experiencing possibly the start of my period? I am still nursing so I wasn’t sure about that.