How often does it really happen?

Our only form of BC is condoms. The only reason I think it may have failed is if, when my husband started to go soft after he finished, his little soilders squirmed out the top inside me haha. About 4 days ago I was doing dishes with my sweet little 20 months old next to me and I got a super sharp pain very low that lasted maybe 30 seconds (implantation?). My period was due anytime between yesterday and Tuesday and I've been nauseous for 2 days now. Deciding whether or not to spend money on a hpt 😮

Edit: Also, we only put one on at the end, so not perfect use. Precum? Feeling more nauseous now haha.