Everything started one year ago when I started getting bad abdominal /pelvic pain. At this time I started to get drenching night sweats and was very tired. 

At this time I had a random lymph node pop up on my left hand side and had this now for a year. I lost 2 and a half stone but now have stopped losing weight and its just stayed the same.

I am having a PET scan tomorrow but I don't understand why a PET scan as I thought that was used to diagnose a stage of lymphoma and not lymphoma itself? Also I have an enlarged right tonsil which has been up for 6 months now and difficultly swallowing. I saw a hemotologist but they didn't look in my mouth or anything. My neck lump is only about 1.5 by 2cm.

I have tonnes of nodes varying in size down my left hand side of neck.

Please can anyone help as to whether this is common of lymphoma and why I am having a PET scan? Also will a PET scan show all cancer like of the tonsils?

Thankyou so so so much in advance.