Need some positive stories!!

Hi Ladies, 
I need some advice and reassurance!! This is a LONG story so grab a cuppa! So was home in Ireland over Christmas and was having pain in my chest. (I had just completed a 10 week metafit bootcamp before I left) had put it down to getting back into exercise properly and probably muscle pain etc... Went to see my fabulous oncologist and he said that I should do MRI and CT scan just to rule anything out! Scans came back all clear except for an enlarged thymus gland (which was also enlarged in 2017 before I started treatment so probably just my genetic make up) So he took it to a team meeting and 7 other said it was insignificant change and no need for further investigation. BUT my onc is very thorough and sent me for PET scan yesterday. He rang me this evening to say the thymus gland is all good but there was a slight yellow colour under my left arm (cancer was that side also axillary clearance on that side) He is now bringing it back to the team on Thursday to see if we need to do an ultrasound or biopsy!!!! So finally my questions....... sorry if they seem like stupid ones but I’m shitting myself! if the other scans were all clear is there still a chance this is cancer??? What else may it be? I have cording in the armpit and down my arm would that light up with the tracer? Is it always cancer if it shows colour?? Does scar tissues light up?? I’m a mess because 4 weeks ago we were celebrating 2 years clear and now this!!