On Thursday evening I gave Grace the shot of MTX.
This morning (Tuesday) I look at her leg and she has what looks like a red swollen bug bite over the injection site. It's about dime size.
My daughter has had bruises from mtx but never the kind of bump you're describing. However, she often had that sort of bump after Humira and I know injection site reactions are common for Humira. Perhaps they can also occur with methotrexate?
If so then this is similar to a Humira injection site reaction. It's a little raised and itches. Do not scratch it because that only makes it bigger and more itchy. Try Benadryl if she can take it (always went away after I took 2 Benadryl at 25mg each). When I asked the pharmacists about it they suggested that I leave the syringe in for 5 more seconds to see if that helps to reduce the possibility of having a reaction (they said it was caused by the medication not clearing the needle entirely).
If this isn't the right image/reaction then contact the pharmacist and describe it to them (or show it to them if you can). They may have an answer for you. Hope she feels better.
Edit: When I got a reaction from Humira it took 1-2 days for a reaction to show. It's possible that a reaction for MTX could take longer or there was some other factor involved.
You need to let her Rheum and Allergist know about the reaction given her allergies/asthma history.
They can tell you how to proceed and what medicine if any to give to her.
DS had injection site reaction to Humira similar to his reactions he has to his allergy shots.
We were able to avoid pre medicating with steroids by changing the injection location.
His doctors discussed the possible issues and determined a plan.
He was also allergy tested for humira prior to anymore injections.