What part of your ear is feeling discomfort? Inner/middle ear or outer? It's doubtful (in my unprofessional opinion) that you're inserting the IEM so far that it's touching your eardrum. If it's really uncomfortable you should stop using the IEMs and possibly see a doctor. If it's your outer ear that's feeling the pressure, chances are the IEM is just too big for your ear (I recently picked up the RHA T20i and it puts pressure on my outer ear).
The way I know I've got a decent fit/seal is when the bass is both clear and full and the rest of the sound is balanced. If the bass isn't full, chances are you need a different diameter tip. If it's weird and boomy, you might try a different material-- my experience is that some silicon tips can create a vacuum, which can also cause discomfort in my ears and a bit of ringing.
Consider that your ears may be ringing and feeling pressure for another reason, too. Earwax buildup, exposure to loud noise, allergies or a cold or general congestion can all lead to ringing and pressure. My allergies have been causing me to feel pressure for the last few weeks.
Also, make sure you're not listening too loudly. IEMs are designed to block out a lot of outside noise and so don't need to be turned up so loudly, but it's also easy to over do it because they sit so close to the eardrum. I don't know what sort of headphones you're coming from or what you're IEMs are, but 'phones with very little distortion tend to sound less loud than ones with distortion, so it's easier to listen to very loud sounds without even realizing it. Start by inserting the IEM with your music off, if that's not uncomfortable then slowly start to turn up the music to a comfortable listening level.