New here and in need of help

I’m a 21 year old female dating a 45 year old deciding to try for a baby. The first month nothing happened period wise but a lot of back pain. The second month there was spotting and my period was three weeks late. All the tests we’ve taken were negative. Is there something different we can try? We were hoping to try to be pregnant by Christmas so a framed ultrasound picture could be the Christmas gift to our parents.