I didnt know that Methortrexate was in the same class as 6mp & Azathioprine and that makes sense now that I think about it. I've gotten sick while on both of those, similar symptoms to the Azathioprine and not as bad as the 6mp. I was up all night vomiting while taking the 6mp. I guess that class of drug just isn't for me.
Methotrexate is similar to 6mp and azathioprine in that it's considered an immunomodulator at the doses used in IBD. But it's chemically different from 6mp and aza and has a different mode of action.
Unfortunately methotrexate does often have side effects, with nausea being a common one. Often GIs prescribe it in shots instead of pills, as that often reduces the nausea for patients. You could ask about
that as Curious Crapper suggested. Also, as mentioned, folic acid supplements are usually prescribed as well when one is taking methotrexate.
There isn't a whole lot of data on its use in UC, as said above. And even for Crohn's, there's only limited data. That's not to say that it doesn't work, just that it's not so well known how effective it is.
It's often used when one needs more help from a biologic but the patient can't tolerate 6mp or aza, so that's your case.