I dont think weight gain is a side effect of aza, but once you start to heal and feel better in yourself its more than likely you will eat more and absorb more through your intestines, making you gain weight.
You should have regular blood test when you start aza (I think it was weekly for the 1st month, fortnightly, next month, monthly for a few months after that then 3 monthly until you stop taking aza). So as long as you can fit your holiday in within those dates (and you feel well enough) there shouldnt be a problem.
I'd suggest researcing your holiday destination to see if you can take all the drugs you are on into the country. Keep them in your personal luggage as you wouldnt want them to go missing, and keep a repeat prescription just in case you do lose them and need to get some more. Also remember to take more than needed in case you get stranded for a few days