Just an FYI I am really really sore from polo yesterday - never been so sore after riding!!! I have already had 3 baths in the last 12 hours, and walk worse then a little old lady!
It is such a good work out, I love it, every time I play I am stiff!! Like you I am NEVER stiff from riding, I think its because you are so active playing polo, you are leaning out and using your core a lot more, it is great, so much better than going to the gym!!!
Haaaa - I felt like that yesterday after not riding for 3 weeks.....keep stretching out your legs - calf stretches and best one for riding (and squash) is to pretend you're going to sit in a chair then cross one ankle over your knee and gently press down on the knee of the leg that's out at an angle (god, that sounds complicated but it's not honestly).
Failing that, go down your local massage parlour and pay $20 for a massage....