Symptoms: red swollen vagina, constant urination and pain at the end of urinating. knowing that tests came negative for uti. what could this be?

Could be: It sounds like you have inflammation from some cause - could be infections (bacterial vaginosis, severe yeast, stds), or could be a bladder condition, like interstitial cystitis which can cause the bladder pain, but won't cause the redness and swelling of the vagina. You could try soaking in aveeno oatmeal baths (powdered colloidal treatment put into bath water) to soothe the skin, but may nd dr.