Cramping for 2 weeks, period 6 days late, negative HPT, lower back pain...?

You may just be stressed out. Or have you been on any antibiotics? I went on some for a UTI and I had those same symptoms. My muscles were sore and always in the bathroom. Did you mean that you haven't had a period for 3-4 months? If so, that could be related to stress or a hormonal imbalance. Check with your doc and see about getting on some medication if needed.