Thanks for providing quick information.
Well, sudden development of bruise on thigh should not be left ignored, and have to be clinically checked by doctor. Possible causes are-
Your doctor can advise few blood tests for evaluation and you will be advised treatment accordingly. You need to undergo-
-CBC (Complete blood count).
-Coagulation studies (PT/PTT/INR).
Sometimes, small bruise develop by its own without any reason, and may resolve by its own.
If you develop multiple bruise, or symptoms doesn't improve in 1 week, then you should be seen by your primary doctor. For the time, use ice packs on affected area. You should keep the bruised area raised above the heart. Usually small bruise will get resolve by its own.
I hope this helps.
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