What you report is a little unusual but not unknown. We have come across patients who have been treated for several sessions and only then shown a reaction to needles. Sometimes there is an obvious cause - change of needles, slightly deeper insertion in muscular areas - but most often it is an unpredictable reaction. The only concern would be if the blister/bite like bumps were a potential source of infection if they became open, or if they increased in severity. At this point a practitioner might just refer on to a GP for an onward referral to a dermatologist. In the vast majority of cases, however, this tends to be a short-lived phenomenon.
The other issue to consider is whether the reaction is not a physical one but a sign of some kind of energetic release. It is very much a part of Chinese medicine theory that some of the physical problems which affect the body are the result of pathogens entering the body or pathogens which have built up in the superficial tissues because of blockage or excess. When treatment 'bites' this can sometimes cause the skin and superficial tissue to become quite reactive, as well as in some cases generating rashes and areas of heat. If this is a treatment reaction it will be short-lived; the majority of what we call adverse events are transient and minor.
The best thing to do is to ask your practitioner if there has been any change of equipment or style from the first to the second session, and if so revert to the former. If not, then it is simply a matter of keeping an eye on the small bumps to ensure that they recede as swiftly as they came. We have come across patients in whom this is a benign and continuing reaction to treatment, and as long as it is managed with common sense there won't be a problem.