Missed period

I missed my period last month. 
​I had it in June on the 21st and it lasted about 5 days. Missed it in July. I was sexually active in July around the 4th. 
​I started heavily excercising and I was stressing over summer school finals. When my period should have came in July. 
​After my missed period I took an equate (wal mart brand) pregnancy test twice. Not pregnant. 
​I still haven't got my period yet. Should I be concerned? Is it a possibility I could be pregnant?