I'd say similar to bednobs and retrochic, try and get in touch with the nurse or, if you can't get hold of her try the secretaries in your rheumatology department and see if they can arrange for someone to give you a ring back or advise you of the best course of action.
Some departments have a helpline you can ring, though if like mine you leave a message and it can be up to a couple of working days before they can get back to you.
I've had to keep off my biologic injections for a couple of weeks as I've got a bug I can't get rid of. Seems to be doing the rounds round here.
Keep in contact with your GP though in case you need any support with your chest/breathing. It's not a bad thing to have a regular GP who is up to date on your condition (medically and personally) and can support you where possible from their end.
It can be a pain of a drug where infections and such are concerned, especially at this time of year with lots of colds and bugs about. I never got on very well with it but it works great for some people and is a good base drug for other drugs if you can get settled on it.
Mtx can cause chest issues but, as bednobs says, not usually serious complications so early on, more likely a bug, infection or virus which is not helped by the mtx.
You need to speak with your nurse/consultant about it if it is causing you problems generally.