As for our back pain issues, Ruby, it might be that your hamstrings are getting tight, either for lack of conditioning and/or because your bike fit is off. Tight hamstrings are notorious for causing low back pain on the bike. Next time you're out riding and your back starts hurting, stop and gently stretch your hamstrings fora a few minutes. If that helps at all, make a point of stretching your hamstrings both on and off the bike.

If strething helps but overall you're still getting tight hamstrings, you might consider tweaking your bike set up, but at that point, I'd recommend seeing a fitter.