Is this discrimination and what should I do?

I recently got a job at a facility that makes industrial chains and conveyors. Whatever the outside temp, its usually about 30 degrees warmer inside. Lately its about 80-85 degrees. The work I do is very physical and its difficult to stay hydrated and cool.

Because of the type of work there are only 5 women employed at the plant including myself. The other 4 women have been employed ranging from 1-8 yrs. All of the men wear tank tops, tshirts with cut off sleeves, and many even cut large areas out of the sides of their shirts in order to stay cool. 3 of the women wear tshirts/ tanktops and 1 cuts her shirt out on the sides and her bra is showing. As summer sets in its getting even hotter in there so I began wearing a tanktop with low sides and a sports bra underneath. A couple times I even wore a spaghetti strap tank top with a high front so there would be no cleavage or anything showing. I figured as long as my lady parts are covered it would be fine.

Somebody complained to human resources that my attire is inappropriate and now I've been told that I can't wear spaghetti strap tops or any top that shows any part of my undergarments. These other women have been dressing like this for years and there was never a problem. The only difference I can see is that they are bigger ladies and sort of manly while I'm petite. Its not possible for a girl to wear a tank top and not show some part of a bra. I feel like I'm being discriminated against and its unfair. What should I do?