what's wrong with my tetra?

Do you have a picture of the red marking? Is she bloated or have red appearances anywhere else, like the tail? Is she swimming odd? Eating? Did her mouth also whiten with the appearance of the red?

In this case it sounds like possibly a bacterial infection, or fungus, or potentially an injury.

(she may just be constipated too....if she eventually gets a long dark stringy excrement, then that is what is was, conversely, if she gets a long stringy white excrement, it is an internal bacterial infection)

Oh, and I almost forgot...

Did you check your water parameters????

LOL ;)

(I am amused at your reference to water parameters. It would be a normal question because of how important water quality is on the health of the fish and would be a valid question especially since you did not give much detail about her ailment.)

How is asking for water parameters pathetic? It could be poisoning from toxin buildup. Either way, it could be an injury if it suffered trauma to that area. Could be an intestinal infection. Do you have any aggressive/territorial fish in your aquarium?

do you have an aipump? how high are your nitrates, is your ph at 7.0 or lower, if it is not, all of your teras may be affected buy this issue, they can tolerate higher PH but if there was a sudden change, or if you did not carfully aclimate them before adding them to your tank, there can be issues, tetras are very sensative to water quality. if she has a wound, she will never recover in poor water condition. you can add melafix, but like i said, check your water parameters, it is very important and should be done every few weeks. minimum, more if you are having issues.