Pregnancy or stress? Help!

Patient: Hello, I need a answer fast. I had sex with my partner August 13 or 14. I got a uti and went to the doctors August 16th and i got urine tested for pregnancy and it was negative. Than I missed my period not knowing it was due to my uti and i tested myself again 17th and it was negative. Than my period came August 19th and it was bad, sore breast, nausea, cramps, really heavy flow. Like usual. Than it starts stopping and a light flow. It stopped August 20th and i took another test and it was negative. Why was my period so short? I was going through some depression through the month in between my period. Could that be the reason it was so short? Could i be pregnant? Could it be implantation bleeding? Could the stress triggered it all?