Gosh, that sounds awful. I'm so sorry you have to deal with this stress.
First of all, I hope you were officially placed on FMLA when you first had to leave and since you've returned to work.
FMLA is an Act which ensures that people's jobs are protected for 12 weeks of leave, in any increments, for a medical reason.
So, 12 weeks of FMLA breaks down to 480 hours of protection. Count up how many hours of work you've missed since the accident and if it's less than 480 then your job is still protected. Period.
Make sure you separate when you wanted to increase your hours, if they are restricting you then those hours don't count towards the FMLA. Make sense?
It sounds like you should be protected under FMLA and therefore your job should be 100% protected for you to return to at your previous level (40 hours a week) when you are able to.
Your HR department should have informed you that you were under FMLA protection right away after the accident and that an injury caused you to miss work.
If your office doesn't have an HR Dept. then whomever fulfills their function should have told you. If your boss does that then he should know this and if he doesn't he's going to get a very rude awakening.
Make sure you have everything in writing! Get a letter from your Dr. that informs your boss that you are to return to work at 7 hour days until further notice. Make sure you get it in writing that they won't let you increase your hours even though you are able to.
This is a very strong Act and you are protected by it. No judge will tolerate this kind of behavior from a work place.
http://www.dol.gov/whd/fmla/
After you are all better and working full time, I suggest you change employers ASAP. No one deserves such an unsupportive employer.