If your rabbits poop normally looks round and not like that, it's very unlikely to be megacolon. Megacolon is a chronic disease, usually genetic. But your rabbit could have some digestive upset or GI slowdown of some sort, that's causing the abnormally shaped fecal poop.
The first thing I try with a rabbit not eating due to suspected GI upset, is the baby gas drops (simethicone) Blue Eyes mentioned. Unless it's been more than 12-24 hrs, my rabbits condition is deteriorating, or there's evidence of true bloat or true diarrhea (not to be confused with mushy cecotropes), all of which are medical emergencies that require immediate vet care.
https://rabbit.org/2013/02/gastrointestinal-stasis-the-silent-killer/
Because meloxicam is a prescription med, it's your choice whether to give it or not. For my rabbits, I may try meloxicam, but that shouldn't be given on an empty stomach. So it will depend if I feel it's safe to give a little food or not (a contraindication with bloat and complete gut blockage).
I'm less concerned about my meloxicam being expired, as it's not a med that usually requires refrigeration (though I do keep mine in the fridge to preserve it better), so being expired may only reduce the effectiveness some. Provided it's also been stored correcty.
If any of the emergency conditions above occur, I try and get my bun to a knowledgeable rabbit vet right away, or first thing in the morning if that's the only possible solution. So that's what I do.
https://wabbitwiki.com/wiki/Veterinary_emergencies
https://rabbitvet.net/AustralianRabbitVets.htm