Any ways to treat a stiff bottom and inner thighs?

I went back to school 2 weeks ago and I have had a stiff bottom and inner thighs for roughly a week. I've done 4 sports lessons during this period and have never had this before because I normally go running and do yoga when I'm not in school.
Are there any suggestions to get rid of this because it's actually quite painful and bending down to get something or bending my legs to sit is really painful.
Any ways to treat a stiff bottom and inner thighs?
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