Any ideas?

I am a week late but have taken 3 hpt and they were all negative. I’m not having any symptoms apart from being tired and lower back pains (if that’s one!) I dont think that I am pregnant but could there be any explanation. I came off the pill 3 months ago with periods being regular straight away. I’ve also lost 20ibs since new year. Could these have an affect on my period or should I take another hpt and when? x