My period is about 4 days late. I have taken 3 pregnancy tests, all were clearly negative. I know it's not the brand of test, as I've used it before and know that it's sensitive. (first response early result) I'm super confused. If I'm not pregnant then what is going on? Should I go to a doctor? Any one else experience something similar?

4 days is nothing... and probably too early to get accurate results from a test. Wait longer and see if you get your period the next time you're due.