Illness/Sickness: Odd purple bump

I noticed earlier that my axolotl, Crowley, had a purple bump right beside their leg. I have no idea what it could be and don't know how to treat it.
I've only had Crowley since March, and they're my first axolotl.
I got them at a pet store, unfortunately in an overcrowded small tank with some siblings,all juvies (Crowley was the only one with all legs intact)
All water parameters are normal at the moment, but over the weekend there was a spike in temperature (went from around 20 C to 24 C) but it's back to normal now.
Their diet's mainly bloodworms or earthworms depending on what's available.
They've never had a negative reaction to their diet and aside the temp spike over the weekend, the water has always been optimal.
I'm wondering if this might be a reaction to the heat spike, but I'm not sure.
Please help out if you can!!

Can you get a better picture? It might take a helper - one person to coax Crowley near the glass in a good position and the other to take the picture.

I am hoping it isn't anything like this http://www.caudata.org/forum/f46-be...lanoma-cancer-axolotls-reference-picture.html

Good luck!