Could my mare have back problems after having a foal?
Nice short answer - yes. Horses also frequently have body issues regardless of foals.
had the foal perhaps 2 months after she was ridden by her 8 year old regular rider she spooked and the little girl fell off! She then went absolutely crazy( nothing nasty she was just in a major panic)
That could potentially be if her foal wasn't there/she wasn't allowed to be with it. I would always ride a mare with a foal at foot... with the foal at foot.
She said she started spinning on the spot, my friend kicked her forward she took 2 steps forward did a little bronco then reared..
Sounds like she could have stressed at being alone with a stranger or such, then your friend kicked her... if she reared, there's also a fair chance whatever your friend did next caused that too - rearing under saddle is commonly due to too much rein pressure.
She did not put one foot wrong! ( but is that because I was there the whole time?) I'm so confused asks to wether she could have a sore back?
Well unless you're a chiro or vet, it's not surprising that you're confused, so I suggest you get someone like that to check her out.
I'm abit tall but she's 13.1 and I'm only 8 stone s
She won't care if you're too tall for fashion
& you're far from too heavy for her size.
If after you have her back checked anything physical is ruled out, I would suggest hiring a trainer/instructor to work with you & her to get an idea about what problems she/you have.