Hello, I Had Identical Twin Daughters On 11/08/13, After Giving

Hello, I had identical twin daughters on 11/08/13, after giving birth I chose to use nexplanon as my birth control and I was never regular before or after the implant.. last night I noticed when one of my daughters crawled up on me that its painful (like a knife inside) surrounding my belly button when pressure is applied also it is firmer then my sides .. I'm worried because nexplanon is supposed to be 99% effective but there's still the 1% and I read that because of the hormones home pregnancy test won't work... thanks for your time