Bike Fit Question.

Alright, so I had a bike fit prior to getting the Kumo. Obviously this is for a road bike and I am very happy with the fit. I’ve since used it and adapted it for both my commuter, track bike and another road bike… All results have so far, have been fanstastic.

Now I’ve started riding my mountain bike again in preparation for this years MONT 24, and after about 35-40 minutes of riding my lower back starts to ache and the muscles get tight, mostly next to the spine.
This has been the first lot of riding with the new cockpit setup. I’ve come from 685mm bars and a 90mm 0degree stem to 100mm -10degree stem and 640mm wide bars with similar sweep/rise to yhe previous bars. I can’t remember and real discomfort with the previous setup.
The current setup up I’ve got measures with 5cm less drop, 2cm shorter saddle to bar length and the seat height is 4cm less (to give my nuts a little room on techy sections).

Now: has anyone who’s had a road fit adapted it to there mountain bike? Any suggestions welcome before I start experimenting with trail and error.

Have I just got shit core strength/being a bitch?

How does your fit compare on the mtb in relation to the bb?
How far are you behind the bb on the roadie?
Also 4cm is a lot to drop your saddle by, I have no idea on mtb fits but if I droped my roadie saddle 4cm I’d be all sorts of trouble.

Does the back hurt on the mtb even if your on the road or bike paths?
If not it could be a non bike fit problem but a instability/core problem.

Have you been riding on the road a lot lately? Your new fit and more Ks might have built up new muscles that are tense/tight in places they weren’t before. Could be worth a trip to a physio / learning some stretches.