Thank you for writing in today. Based on the information you have presented, I think I can offer you some assistance. However, first of all, I have to mention that you should check with a provider, since this does can not be a substitute for a patient visit. Now, with that being said, I will continue.
Based on the information you have presented, I suspect this is more of a vulvovaginitis, rather than a urinary tract infection. Vulvovaginitis can cause some burning, due to an inflamed urethra. Vulvovaginitis is caused by a number of things, including sexually transmitted infections, yeast infections, bacterial vaginosis, vaginal atrophy, and irritants. Now, let's look at these possibilities more in depth, as they pertain to you.
Sexually transmitted infections. You deny risk, and you would likely have additional symptoms, such as discharge. Finally, this would not explain the pattern before your period, so I suspect this is not the cause.
Yeast infection. You would likely have an obvious, yeast like discharge and odor. Also, this would not explain flaring before your periods. A yeast infection would likely persist (not be cyclic) if not treated, so I suspect this is not the cause.
Bacterial vaginosis. This would have a strong fishy odor and discharge. This is probably not the cause.
Vaginal atrophy. This occurs due to loss of estrogen, usually due to menopause or hysterectomy. This is not an issue for you, so probably not the cause.
Irritants. These are behaviors, chemicals, environmental, etc. factors that can cause some vulvovaginitis. You have many of these factors. You are an avid exerciser, frequently exposed to sweat and tight workout clothing. In addition, you do not change clothing right after working out; therefore, your vagina is exposed to sweat (very irritating) for a prolonged period of time. In addition, since there is a bit of a cycle here, just before your period, your hormonal changes may be making your vagina more sensitive to these irritants.
In your case, I could recommend a few things to consider. First of all, you may want to consider discontinuing the things your are doing now. Consider sitting in a cool bath several times a day for the next few days. Consider using a bulb syringe to wash your genital area with water every time you use the bathroom. Until the burning resolves, just use water to clean the genitals. Then, start using a mild soap when the area is more comfortable. Consider changing to cotton underwear and avoid thongs. Consider wearing looser workout clothing and showering/changing right after working out. No bubble baths. Try to keep your genital area clean, dry, and able to breathe. I suspect this may really help.
I hope this helps. If you have any additional questions or concerns, please let me know.