RE the fit:
It is possible more load is leading to heel drop. As you push bigger forces your body recruits different muscles and assumes a different position. This can lead to pelvic rock and general pain from back muscle engagement should there be an impingement, weakness or issue with position.
It can be exacerbated if cranks are too long for you or the fit (longer cranks can allow less reach/tilt due to closing hip angle at top), especially if a fit is transferred from say a road bike to MTB (road may have shorter cranks).
Another possibility, leading on from the positional thing, is the reach or angles are not right. It is typical for the torso to shorten under higher loads, this can additionally change pelvic tilt. Reach, your back angle, bar height and bar shape can impact this.
Personally I would do a chunk of core work if is totally pain free, but would also revisit the fit (ideally in the same state where you have an issue - i.e. if pain worse when tired ride to the fitter and put the miles in first).
All fitters are not the same and it is so disappointing when people say ‘get a fit’ without qualifying it. I have had two poor and inappropriate fits over the years. Both were Retul qualified (i.e. paid to go on a 3 day course, if you are feeling cynical), one with a sports science backgrounds and Retul Level 2 (TT/aero). Both got saddle height spot on but no other aspect of the fit worked for longer rides.
Third fit changed almost nothing on saddle position but big changes on front end and loads more comfort. She has a physio background, rode herself and now works for one of the most prestigious fitters in the UK - not a surprise to me but is a shame she is no longer local!
I would also politely suggest a physio rather than a chiropractor. An osteopath has worked wonders with me in the past but my personal opinion is neither is a replacement for a physio, especially one who rides or has fitted people on bikes themselves.