Bit worried about my sow Flo.

She was weaned from her piglets when they were about 6.5 weeks old, 4 gilts were reunited with mum about 2 weeks later. She had dried up, and her teats had shrunk back. Now they have all been back together for about a month. I have seen them trying to have a suckle on her but presumed that she wouldn't let them. OH has also seen one piglet trying to suckle.
Over the past few week, one of the back teats has been looking a bit bigger (this teat was always bigger and she was a bit lopsided with her teats while suckling due to only having 7 piglets).
Anyway, tonight, I noticed one nipple looked quite sore as if it had been bitten. Then I had a feel of the big teat and it is quite lumpy. It doesn't feel hot and she is otherwise completely normal. Is she producing milk again or could this be mastitis? What shall I do? Would it be a good idea to separate her again?
Advice appreciated.