wasn't there an opportunity here to negotiate the terms a little ?, i.e yes we'll work on, but he/she would let you start/finish at a different time later in the month in keeping with the hours you had worked, to the sole benefit and discretion of you.
you are absolutely correct in that you were under no obligation to do any additional work, and the manner in which it was put to you was pants, but in life you soon realise you get where you want to go by being "smart", if you get into the habit of "I only get paid from x till y", then you may be an unwelcome and expendible commodity in your future bosses eyes.
many people have posted "why bother for a PT job", conversely use everything in life as a learning experience and realise your manager is a clown who doesn't know the 1st thing about man management
in answer to your original question "am I in bother", If they want rid of you they will no doubt find a way as there is less protection for part time employees, additionally you could make the situation worse by how you handle any subsequent disciplinary meetings. If you value the opportunity to make money, then stay tight lipped but suggest the way in which it was dealt to you was unprofessional and antagonistic hence your reaction. In future should the situation again arise you would hope it could be done differently.