What Causes Menstrual Cramps, Nausea And Lower Back Pain With Delay Periods?

Hi, I have been taking 2400mg ibuprofen a day for about 3 weeks, as prescribed by my doctor for my back. I am a week late for my period, but have very bad menstrual cramps, nausea, and lowerback pain. My boyfriend and I haven't had unprotected sex,but we did have protected sex, but I took two pregnancy tests just incase, they both came up negative. Could I still be pregnant? Or could the ibuprofen be delaying my period?