Metformin ER side effects

Ok I know the general side effects of metformin when you first start, diarrhea. I did have diarrhea for the first week on met and it went away. No I am going on week 4 and I ate so badly over the weekend and yesterday I almost threw up, had diarrhea and felt so sick alllll morning and all day. Then I started to get back aches. I slept over 12 hours and I feel better and not so nauseous but my back still aches so bad! I don't have a fever or chills or caugh/runny nose so not thinking it's the flu. Thinking it's a bad side effect for taking met and eating horribly? This happen to anyone else???