I noticed a little under a month ago that this was happening. When I get aroused my inner labia and vaginal walls swell. I've noticed it now three times. The entire time It's been happening, I haven't been sexually active with anyone and I'm not rough on myself so simply being raw couldn't be the answer. The first time I saw it I thought maybe I'd just over done it a bit on myself and I wasn't worried. I didn't investigate much so I only noticed, at that time, the Labia swelling, i didn't check to see if it was inside as well. I avoided arousal for a couple days and when I next had a bit of alone time I paid attention to see if I would swell and I did. I still wasn't aware of the inside swelling so again I only saw the inner labia swell. I don't have to be touched there for it to happen, just physically become aroused does it and the more turned on I am the worse it seems to get. The third and last time it happened was last night. I had given myself a break of about..five days to a week to try and see again if maybe that would help. But it still did it. I also found out that time that lube making contact with it burned so bad I almost cried. That is not normal at all, the lube i used i have always used and had no problem with it. That time was when I realized my vaginal walls were swelling too. the swelling was so puffed up I could barely fit a finger in and that hurt. It made me very concerned. The last I had sex with someone before this happened was about a month before hand and it was safe sex with someone I had already had sex with a few times before and had no issue with this king of thing. I've never had any genital issues other than a small yeast infection when I was much younger. I am 18. The swelling has completely disappeared since last night.
It is normal for the inner labia to become engorged as part of the arousal process in the same way as a penis does. The purpose is to push the outer labia apart in order to better expose the vaginal entrance.
There will usually be an increase in sensitivity as part of the arousal and resulting engorgement.
I speculate the soreness you are experiencing is a permanent thing but you are mostly unaware of it until that increase in sensitivity.
Clearly you need to see your doctor.
Practicing safe sex - using condoms is only part of safe sex and only reduces the risk of infection and pregnancy rather than eliminating it altogether. It still needs vigilance and a careful choice of partner as part of practicing safe sex in order to further reduce the risk to acceptable levels. Some infections and infestations cannot be prevented although the femidom provides distinct advantages in protection of that type.
It is possible your soreness is being caused or aggravated by an intolerance or allergy which can be developed at any time. Detergents, fabrics, lube, latex - the list is endless. You might need to begin changing things in order to discover what is causing the soreness if doctor checks are negative.
Until then be sure to finish bathing by rinsing with clear water. Wear loose clothing for good air circulation.