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If you are feeling heat and seeing noticeable swelling, he might have a strain or sprain and he should not be worked until the symtpoms resolve.
If you do not feel any heat, he may be having an allergic reaction to a bee sting, bug bite or other allergin. In this case, you can work him if he has no other symptoms.
If the symptoms do not resolve within 24 hours, you need to call your equine vet and arrange for a visit.
Let me know if you still have questions.