Hi I Been On Metformin For 4 Years Off And

Hi I been on metformin for 4 years off and on and I always gotten my periods every months except for when I got pregnant so in April I had a three day period and the forth day was just spotting now this month I am five days late I have tested twice and it was negative what else could be preventing my period from coming on am I testing to early ? Can I still be pregnant I used two different brands in test