Health first do you have prior shoulder injuries or have you worked out, made home improvements or do manual labor or something that strains the shoulder already before throwing? Adding power to a throw and not being used to it can easily strain the shoulder alone but i would try to stay as loose as possible muscle wise as late into the throw as possible at least the left pec position initially to make sure you're not strong arming. Did you warm up? If you did and was throwing mellow you might have shoulder issues that you are not aware of yet or you could just be stiff from hunching over a computer sleeping in a twisted position or laying on the throwing shoulder so ergonomics when not throwing matter too. Nothing beats warming up and stretching and starting out a low power for pain prevention. Everyone needs to find out how many throws they can do before aches set in. My record for one day was a little over 1000 drives at full tilt and i've heard people recommend 50. No wonder i was in pain for 5 days... Repetitive stress injury potential right there. I had done a lot of field work conditioning me up before i did that.
It is normal for balance, timing, body part placement=muscle control and sequence of movements to go haywire when reaching back. the subconscious takes over when it detects a chance of falling down and bye bye control. False alerts happen and with practice the conscious mind can make you stay upright after a wild throw that the subconscious thinks is too dangerous. It can take months to learn control and condition the muscles to be able to allow you to be in control. The stronger and more coordinated you are the easier it is to learn reaching back far.
Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.