As you have told that you are taking methotrexate
for RA ?, SLE ?
Prednisolone used for less than 2 months does not increase any significant weight
Besides you are using prednisolone in tapering dose , so not to worry about the weight , just complete prednisolone course , as advised by your doctor
Methotrexate and prednisolone donot interact with each other and are safe to take together
Hope this helps