It's possible your "irritation" is a herpes break-out because you mention that it is ALWAYS in the same spot on the left base where the condom meets the penis. A herpes virus can be inoculated through the skin by skin to skin contact. From oral-genital contact (herpes types 1 and type2) and from genital to genital contact (also herpes types 1 and type 2). Although, herpes 1 is usually found in the labial area (lips/face) and type 2 is usually found in the genitals. However, this is not always the case. And, as I said previously, herpes type 1 is common in most people.
At the place where the virus is inoculated, the herpes virus DNA attaches to part of the person's DNA and forms a capsule around itself and then it moves on down the person's nerve to replicate (clone) itself within another part of the person's cells and so on along the nerve to the nerve ganglion root and sometimes all the way to the person's spinal nerve root. Then the herpes virus remains dormant (not replicating) until for some reason it will become active and the person will shed particles of the viral DNA. The person might not know they've been infected because they may not have any symptoms. The virus can sometimes cause paresthesias which can include "anesthesia" which means the person won't feel the painful blisters. Sometimes when the virus recurs (recurrent herpes infection) the virus will move along the nerve from the root back to where it was originally inoculated and then it causes the skin lesions which are usually itchy and painful.
Your partner's blisters sound like a cluster of herpes vesicles (see photo above). There are some people who don't have as severe a reaction as other people; however, usually they will feel "itching" or "tingling" sensations which is due to the herpes virus irritating the nerve.
I suggest you both be tested for herpes by having the HerpeSelect and if you have a blister with fluid then have the fluid cultured immediately because there is a chance of a false negative culture the longer the blister is present (over 48 hours). Fresh herpes blisters are usually positive for herpes. Since your girlfriend has fresh blisters now, then this is the ideal time for her to have the blister fluid cultured.
You both should have the HerpeSelect test which may come back negative with low titer levels but that just means that you should wait another 4 weeks and be re-tested. Sometimes, if a person is newly-infected with herpes, they will have a low titer level for IgG and/or IgM antibodies because their body hasn't had sufficient time to build-up an antibody response to their infection. It doesn't mean they're not infected with herpes---just that sufficient time hasn't passed for antibodies to be formed and therefore be picked-up by the HerpeSelect test.
If you have other questions please ask me.
Dr. Hanson, MD, PhD