mastitis is an overproduction cycle which is usually followed by an under production cycle, so my advice would be to express or feed from that breast every time it felt full, until it stops being painful, then again when lo would usually feed from it for a couple of days after to ensure your body pulls through the underproduction part of it.
It sounds like a lot of work but it is important that you constantly empty the breast as (worst case) it could develop into an abscess, then once you've cleared the mastitis, you need to build your supply up again.
if you express from that side and feed from your other side it should keep your supply for your good side up enough (and you can freeze all the milk you express to have some in stock

)
xxx