That could be drought stress or fertilizer burn.

skuiiy: We bought the plant with the shown plastic pot and kept it in there since it seemed to be doing well & producing flowers and fruit.

A plant from the nursery producing fruit is often doing so because it's stressed. When tomatoes are stressed, they fruit because the plant rushes to reproduce before it dies. So assuming it's happy because it's fruiting could be incorrect.

Even if the pot is OK now, it won't be much longer unless that plant is a dwarf or small determinate.

skuiiy: -Gave it liquid fertilizer.

-Added about a spoonful of epsom salt to the soil/mulch mix it's in and mixed it in a bit.

Epsom salt is really only necessary if the plant has a magnesium deficiency. If not, it could cause problems. Also, it's possible you've given the plant too much fertilizer, which can manifest as leaf burn

skuiiy: now I'm watering(about 2 cups of tap water, instead of just 1)every other day starting today to see how that goes.

First: Pot it in a much larger container. Gently knock off as much of the current potting mix as possible so that almost all of the mix is fresh, as this will help correct the problem if nitrogen burn is the problem. Use a normal, fluffy, well-draining potting mix, not a mix advertised as moisture-retaining, and not anything labeled as "garden soil". Mix water into the mix well before putting it in the pot, as peat-based mixes can initially repel water if they're dry.

Second: Don't measure your water or water on a set schedule. To determine when to water, stick your finger down into the mix a couple inches. If it's dry, water and water very deeply, until the water is running freely out of the bottom of the pot. If it's still wet a couple inches down under the surface, hold off. In your climate, when it gets hot, don't be surprised if you have to water daily.

As for fertilizer, you can give the plant a dose according to instructions once a week or every two weeks (depends on how frequently you have to water), or you can water at very dilute strength with every watering (dilute more the more frequently you have to water).