Photo Credit: Active.com
The goal of your footwork on your groundstrokes should be to get to the ball so you are able to make contact between waist level and shoulder height.
Some players seem to like the point of contact a little below waist level but this means the ball must go up significantly before reaching the net (which means it also has to come down) and this will make it more difficult to hit an offensive shot.
If you do a good job getting to the ball when it is at the height I described, you will be able to use your regular swing consistently. A change in your normal swing is often the result of your feet not doing quite what they need to, and your arm trying to make up for that.
Keep working on quick small steps when moving to your groundstrokes and your arm will thank you at the end of the match.