Too small mtn bike = shoulder/back issues...?

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Since DS has taken over my mountain bike I'm borrowing a friend's TINY Salsa Ala Carte until we can manage to invest in a bike for me (I'm really eyeballing a 14"/XS Salsa El Mariachi 29er). It's an XXS/12.5" frame. I am just shy of 5'4". She is about my height (maybe a half inch or so shorter), but tends to like really small frames.

We have the saddle maxed-out and I could still use it a hair taller. That's not the big issue. When I ride it I feel sort of like I'm on a clown bike. I assume a 14-15" frame would be ideal for me (30" inseam, average torso/arms). My Cannondale mtn bike is only an XS (I think the seatpost is just under 14") and is also small for me, but less so. We'd have our DS ride the Salsa, but it's a LOT more bike than my Cannondale. Everything about it is faster and it's a handful. The Cannondale is much more forgiving.

Anyhow...DS and I rode 11.5 miles on Sat. Most of those miles were on the paved rail trail to get out to some singletrack about 4 miles away. Ever since then my back has been p!ssed! I am not really prone to back issues. Sometimes I will sleep funny (or a cat will steal my pillow) and wake with a wonky neck or shoulders, but this has moved from the middle of my back and outwards and downwards. I'm suspecting the tiny bike is to blame, though I know my core could be stronger (I've been doing 1-2 core workouts/week for about a month, now).