I know nothing more about Sinclair than I know about you - nothing. So, I'm looking at this pretty neutrally.
I have the sense there must be something more to this story.
* What, specifically, were you told when you called? Were you told you were supposed to be paid or were you given a reason for non-payment?
* Is the scheduler still scheduling for them?
* Is there a chance that you did more shops than what you were eligible to complete?
* How long has it been since you last sent an email inquiry?
From all you have stated, it sounds like they owe you money. The first thing you need to make sure you have done is printed out all the emails, assignment sheets, etc. on which it states that you have been contracted to do the work and have submitted the work. I'd give them another week - to get passed the holidays - and then send a certified letter. Include copies of your paperwork and ask for either explanation for the non-payment, or the payment itself. If you get no response or if the response is not satisfactory, send another and mention small claims court.
Hard work builds character and homework is good for your soul.