There is a bit of a trial and error (for lack of better expression) period when going back to the gym with diabetes. A good practice is to measure before and after for a couple of weeks so you know how it effects you personally. Eventually you'll be able to find the perfect timing to when you can workout in relation to your last meal and injection.
Different types of activity can effects your BG different both during and (long) after. Some research suggest doing a short 10 second sprint after a longer period of moderate intensity exercise to oppose the drop in BG.
It's a bit more of a hassle to workout with diabetes but don't let that put you off, the benefits of it far outweigh the bit of trouble.
Lastly, good job on going back to the gym so short after your diagnosis.