Hi and WELCOME to YT
-- glad you found us, wish it was under better circumstances. Hmmm...this is a tough one bc it's so hard to say what could be going on here. You didn't switch foods or treats or anything, right? The only different food thing was that pumpkin?
A rock hard and swollen tummy is indeed a concern bc you worry about obstruction, in the worst case scenario. But the xray didn't show that? Did that tummy xray include the intestines, I assume...? Hopefully it did.
If xrays all prove clear, then you do wonder about the "slipped disc" comment - and if it were me, I'd need xray proof of this condition bc you need to know where the disc problem is and how bad it is. If it's causing severe pain - vomiting would not be uncommon, and the tension from the pain could cause both some distension in the tummy as well as bad constipation...but the lax should help there. For disc issues - you have a good regimen - my Wylie has disc issues and when he has a flare, he is given pain meds, steroids, and muscle relaxants....you may want to try adding the relaxant and see if that gets you anywhere. My vet says that this regimen tends to work very, very well on about 85% of dogs - the remaining dogs where it doesn't work may require surgery
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Another option is to consider a 2nd opinion. Again, it's tough to say if this could be related to something else entirely here.