Boston Terrier-sudden lameness / injury?

Question
Greetings Dr. Fry-and thank you for volunteering your time.
We have a 7 y/o F spayed Boston Terrier, 26lbs.  She is incredibly active and physically fit.  This evening she came limping into the living room putting little to no weight on her R foreleg.  On examination, she does not yelp or exhibit any discomfort on varus/valgus stress of either her "wrist or knee" joints.  There is no crepitice on any portion of her limb, and there seems to be nothing impeding her foot pads or claws.
Of note is a "firm" somewhat mobile nodule, middle 1/3, on the lateral (outside) of her R foreleg between the ankle and knee.  Palpating this structure does not appear to cause her pain.
After a bit of snuggle, she did gingerly place weight on her leg and came for a treat.
 Carpets in our house are few and I feel this could be a strain / sprain from a slippery floor-the nodule does concern me though, as we have not noticed it previously.
 For her comfort, I have given her some pain medication(12.5 mg Ultram), previously prescribed, for an earlier condition.
 My medical background is humans...your expertise would be appreciated.  Thank you again for this service.

v/r
Joe
(PA-C/MPAS-Emergency Medicine)

Answer
Most likely a muscle sprain.  If she does not improve with rest and pain medication in 24 hours, then I would suggest an x-ray.  You really should have the nodule looked at by a vet....bostons are prone to mast cell tumors.  They can look just like a fatty tumor but the only way to distinguish is by doing a fine needle aspirate.