I am so sorry you've had this glitch @JenniferKK - your Rad Onc would also need to know about this ....
If you tell your GP receptionist that it is urgent ... relating to your cancer treatment ... they should be able to get you in earlier (most have a couple of 'urgent' spots reserved most days) ... even if you don't see YOUR OWN GP - just one in the practise!
Given that tomorrow is Friday - you don't have a big timeframe to play with.
It would appear to be infected - and pus is the yellow goo .... if you can't see your GP/someone tomorrow - I would suggest you just front up to your cancer treatment centre & basically not leave until you have been checked out OR go to your closest hospital and explain that it is the result of your cancer treatment.
Personally, I would not wait until Monday or later. Take care & all the best xx