I Was On The Implanon Birth Control Close To 3

I was on the implanon birth control close to 3 years I had it removed because my husband and I wanted to try and have another baby so far we have 3 already ages 7, 5, and soon to be 3 I've been off birth control for 3 months had no period at all started taking birth control pills in September, since I've gotten a period I've been using the ovulation tracker however this month im late I have taken multiple pregnancy test and they all have the same result (not pregnant)I'm experiencing spotting I'm having cramps and backaches but the pregnancy test still reads (not pregnant) is there any reason why or is there anything that me and my husband needs to do additional?