Hi,
It could well be due to your shoes - astro's tend to be the most unforgiving shoes in the world in my experience (I also play hockey!) with little in the way of shock absorbtion at all.
Over pronation is simply where your foot rolls inwards slightly and is very very common. It's hard to assess on yourself but if you get someone to stand bare foot on the floor and look from behind, if they overpronate then it is usually quite obvious - the arch at the side (the instep) appears to be rolling inwards, towards the floor.
If you are concerned, get in to see a podiatrist who can advise you on whether you have a problem, and how to sort it out. Also remember that if you're pounding up and down the astro for hours, the pitch often has little in the way of cushioning (especially the old ones) which can be something of a shock to the system if you were playing/training on grass beforehand which is lovely and squishy!
I hope this is of some use, feel free to contact me if you'd like further information.
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist