Not my child

I’m new to the app so I’m not sure if I’m posting in the right forum but my gf and I have been together a year, broke up 2 months ago and got back together last month but we had sex December 10th unprotected as well as December 13th or 14th but these days we used condoms. These are the ONLY TIMES we did anything. She sent me pregnancy tests and told me she’s pregnant but the lines are super dark and that can’t be my child because she hasn’t even seen a doctor yet saying she’s 4 weeks and I didn’t sleep with her 4 weeks ago it was two. Can some women please explain here calculations or something because I’m arguing with her and I need for women who know anything about babies or pregnant woman or pregnancy test lines and dates to tell me exactly what I feel I may be right about so that I can show her what other people are saying because there’s no way this child is mines.