Pay scale changed after inquiry into overtime

Have been working for 3 years at 2 different rates. I travel a lot for this job and the travel is required. Every week I travel overtime from 4-12 hours but I have been refused overtime pay since I earn a different pay rate for work done on days I travel to location. I recently inquired about receiving this overtime and was told I would be taken back to standard rate on days I work at different location (a difference of over $15 less an hour) and that when I have to travel above my 40 hours I will only receive minimum wage and no overtime whatsoever. This was only decided upon after I inquired as to overtime. It seems that this may also be a way for them to cause me to quit my job. This will impact what I make by lowering my paycheck by almost 60%. Can the employer do this in retaliation to my inquiry and could this also be construed as a constructive discharge scenario.

Mind you... I have worked this way for over 3 years and am only questioning this now as I've done some reading on FLSA. Employer has flat out refused to pay anyone overtime even when they require us to travel sometimes over 14 hours in one week.

I appreciate any assitance.