My question involves employment and labor law for the state of: TX

All,

I currently have a sunner internship at a large corporation and am paid approximately $25/hour. My offer letter explicitly states that I will earn $37.50 (1.5x) for each hour worked over 40 each week. Since the beginning of the internship, however (mid May), I have only been paid $12.50/hour for my overtime pay. I first assumed it was a mistake and I contacted HR to inform them of the issue. They finally got back to me today saying that the original rate in my offer letter was a mistake and they only ever intended to pay us the $12.50/hour OT rate. This is a sizeable difference as I average 80 hours / week. I also got the feeling that the company never intended to inform us of the change in pay rate until we asked about it.

Considering I signed my offer letter back in March and the change in terms was never communicated to me, can the firm do this? Don't get me wrong, I'm enjoying my summer and am glad to have this experience but the way the firm tried to Sweep this change under the rug rubs me the wrong way. Am I entitled to anything (especially becase I worked for 8 weeks under the impression I was being paid essentially twice as much as I actually was)?