Ibuprofen is sade to take when breastfeeding.
In general, the type of infection that causes mastitis won't be trasferred to the other boob as the infection is within the duct rather than on the surface of the nipple iyswim. Your LO therefore won't be affected in that sense - he may not be as keen to feed from that side as the milk tastes a bit different but it is important to continue trying to get him to feed as he will be more effective at draining the breast than any breast pump.
The main infection that would be transferred would be thrush which could be more likely after antibiotics. I did get thrush as well following one lot of antibiotics and still drink probiotic drinks, etc every day. A BFC told me I should have done that whilst on antibiotics - it may not help but it doesn't do any harm!
With the antibiotics, the second time G was fine. The first time, she seemed to have an upset tummy (she was only 6 days old though) - we ended up taking her to the GP who thought that she had a separate bug rather than it being the antibiotics. The antibiotics can give them strange nappies though, but won't do them any harm.
If you have used the sling for a long time, I'm sure it won't be that - I had just started with the sling so didn't know what it should feel lik and I can tell now if her head is getting into a position where it is causing pressure and recognise what I had done wrong.
HTH