Large lump on Iguanas hind leg

Question
Our 15 year old beloved Iguana has developed a large hard lump beneath the upper hind right leg (about the size of a 50 cent piece).  The vet we knew previously no longer is in practice.  Otherwise he seems fine and can still climb and get around as he always did.  This lump is on the underside of his upper leg near his tale. He still acts like himself but this has us worried and we were afraid to transport him in the very cold weather before. Could the additional calcium supplement we added have any bearing on this?  Help us please.

Answer
Hello Ann,

Well, to have a 15 year old iguana is awesome!
What type & brand of UVB are you using, a fluorescent tube bulb or a compact/coil light?  How old is the UVB?
which calcium did you switch to & how often are you giving it?
So he can support himself just fine the on all fours?  
What types of foods do you feed him?
It could just be cyst if it is hard, or it could be a calcium deposit.  Normally calcium deposits occur more commonly on the rib area, or cervical column or joints.  Is this on or around a bone or is it on soft tissue?
I doubt it is an abscess as they are normally soft.  Cancer is always a possibility as well.  
It should be aspirated though to see what the fluid consists of.  

Tracie