Arm weakness

Hi, i have been on a long toss program that my pitching coach put me on, normally my arm is fine after long toss, my arm isn’t ever too sore. But the last time i played long toss i just couldn’t throw as hard as i usually could. i noticed that when i threw harder the ball went higher and off to the right (i am right handed). i have never had this happen to me before. Could it be that i need to stretch more. Oh and i lifted a little bit the day before.