Could be dry cow mastitis, Dun. It seems a bit early for colostrum.
For checking dry cow mastitis (not everyone knows this): you see a cow with a firm udder/quarter so bring her in - the affected quarters will either feel *much* firmer than the others or have a palpable lump. If the firmness/heat isn't diagnostic of mastitis, roll the teat gently between your fingers (without drawing milk). If you can feel lumps inside the teat it's mastitis, strip the quarter out. Don't touch unaffected quarters.
Use a lactating cow antibiotic up the teat and strip the quarter daily (or 2x daily if you're keen) until only clear lump-free fluid is produced. For most cows this is 2 - 4 days.
The risk period is 2 - 4 weeks after drying off, and in cows that haven't been treated with dry cow therapy. There might be a few observed later than 4 weeks but it would be unusual, at 2 - 3 weeks some cows also have firmness remaining from the previous lactation. Don't strip to check *unless* you can feel those lumps in the teat.
Catch it early there's probably little pain. Left a couple of days longer the soft goop turns ropey and almost every cow will kick and stamp rather than let you strip it out.
The 'thick yellow stuff' would be like thick custard if it's mastitis rather than colostrum, and nearly always has visible flecks or looks curdled.
