There have been several threads on diving. Maybe try and search keywords like PADI, Diving, SCUBA, Open Water, Tioman, Phuket, Perhentian?
The pressure change from the surface to just 5m is substantial, in fact 50%! So getting ear-squeeze is par for the course. If you have swimming goggles on you cannot equalise like you can with a mask with an enclosed nose-space.
The inner ear (in which the pressure change causes the pain) is a bit like a system that usually only requires manual intervention one-way. That is, as you go down you need to help equalise the pressure by (typically) blowing against your pinched nose. If you’re doing a dive and starting going straight to say 30m, you’d hold your nose the whole way down and blow (‘valsalva’) say every few metres, but it is the first 10m that matter most. On the way up, the pressure releases from your ear through your Eustachian tubes and it can usually do this automatically without intervention of any kind. Diving while congested is not going to assist the latter though.
So experiencing pain in shallow depths is to be expected if you’re skin-diving and either not wearing an enclosed mask, and/or not equalising using nose pinch/blows.
Sinusitis is not great if you’re a diver, as it can cause problems equalising ears, and for various reasons you should not be diving whilst on medication. From the sounds of it if I were you it might be worth waiting until you’ve had your procedure done; there’ll just be more certainty. There is nothing worse than having a dive trip coming up and then being hit with a cold (and congestion) right when you are departing, $-ugh!
But in the meantime you can get started on the theory if you like!
You can do the theory in class ‘at home’, and the qualifying dives on holiday. It is called a referral course. Or PADI now have means by which you can do all the theory online via e-learning (hey SMS do NAUI offer this
p.s. Final thought, if you’re really concerned re: your ears then you can simply have a specific dive medical done. It is not that esoteric, in fact our GP in the heartlands HDB estate behind us offered them.