Am I pregnant?

I have endometriosis and am taking 20mg of amitryptiline every night for the pain. I am a couple of days late and am unsure if I'm pregnant. Can amitryptiline cause false positives and if so would it show on 5 tests? I've been experience alot of cramps but no sign of period. My last period was only a couple of days in length so I'm a little confused...surely if that was implantation bleeding the positive lines would be darker? Any advice would be great. Thanks in advance. X