I have horrible discoloring of my teeth that my dentist said came from my mother taking tetracycline while pregnant with me. What might she have had/?

Doesn't make sense. While your baby teeth may have been stained during formation late in pregnancy, the permanent teeth would not.Staining of your permanent teeth would only occur from exposures or events that occur during their formation. Tetracycline given to you might, but its use in kids was minimal after the 60's.
An infection. Tetracycline is an older antibiotic that was commonly used for a variety of infections. It does have the side effect of staining the teeth during development. See your dentist to find out what treatment options might be available to you.
Teeth staining. The staining you have that the dentist described is called tetracycline staining. It is caused from taking the antibiotic tetracycline while your teeth were developing. This is internal to your teeth. You might be able to lessen te effect with diligent whitening or most likely with crown or veneers.
Does it matter? I wouldn't worry about it. There are hundreds of reasons a tetracyclineay have been prescribed. It also may have been prescribed to you, not mom. Most people who have staining from these meds were prescribed them during childhood while the teeth were forming. Why you were given it is anyone's guess at this point. I would put the effort into fixing the discoloration instead (crowns/veneers likely).