I'm a pitcher myself (college), and I'm with Pitcher3. If you
stop lifting now, you risk the chance of losing a lot of these improvements you've seen.
I use a
protein supplement to help myself recover from the workouts a little faster, but you're a little young. I don't know if I'd recommend that kind of stuff to a high school freshman.
I would advise being careful with your workouts--don't push yourself as hard as you were in the offseason. Shoot for more reps and lower weights; a trainer I had even said to go with 30% of what I was lifting in the offseason (which seemed too little to me, but apparently it works--I go about 40-50%).
I'd say focus on the squats and lower body workouts. The majority of your power comes from there anyways.
You're young, and you don't want to push yourself too hard. Try starting off with just 25-30% of your regular weights with the arms, and just see how you feel. With all the throwing you're going to be doing, you'd be surprised what a workout you can get from just lifting a fraction of what you usually lift.
Workout during the season--just be smart about it.