Weird periods or pregnant?

Ch
Does anyone else think that stress affects their periods in different ways? I have been stressed the last 5 months. 
My periods have been all over the place, Iv been stressing that I could be pregnant (I'm definitely not, negative tests and periods) 
Some months my periods are heavy and only go for a 3-4 days , other months i start spotting even before I start taking my placebo pills and then get heavy bleeding for a day and then the rest of the week is light bleeding? 
My periods have been all different over the last few months last year they used to all come on the Wednesday and finish on the Friday or Saturday. Do you think medication (amitryptilin and antibiotics) could cause my periods to be all over the place? 
Surly pregnancy wouldn't cause my periods to be like this , wouldn't  my periods just stop all together? 
Before anyone says take a test I have taken a few tests and they were negative.