Mica,
I think the official term for the gunk is brown goo (my term, that is)

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Be careful being in any sun without sunscreen, especially so soon after treatment. Even radiant UV rays on radiated areas could be a bad deal. When in doubt, use the sunscreen.
I get something like a prickly heat rash on my chest and right shoulder only. It doesn't turn red but just bugs me. It started around March and I can keep it somewhat controlled but not eliminated.
I don't know what to think about the groin pain. You should get in the habit of checking your fever at least daily for a while as your body is somewhat beat down from the radiation and chemo and infection could pop up anywhere right now. If you run even a constant low grade fever, you should get it checked out.
20 days out from treatment I could not drink water because it was too thin. I could drink Boost and milk and lived on both with Ragu spaghetti. It was the only thing I could eat easily so I just went with it.
Try sipping something thicker and see if it is any easier. I still struggle at times with choking on water but I try to drink 100 oz or more a day.
I know how hard it is to keep the morale up, for sure! I describe it as a roller coaster ride that is wild at first but after a bit you know where the curves and drops are so they become somewhat expected and not as shocking to the sensations.
I am glad you are past treatment and I hope and pray for contiued healing and faster recovery for you. Keep up the hydration and nutrition and you are well on your way!
Ed