We would advise you not to worry too much about short term adverse effects after a first treatment. The vast majority of these are short-lived, and we would not be at all surprised to hear that by the time you receive this reply they will have gone. Our experience is that this usually only happens after the first treatment, and you should have no reason to feel any trepidation about the next treatment.
There are a number of possible explanations for this. When someone has had Western medical acupuncture, which we suspect is the case, there may well be some sinus involvement in the pain and the treatment may well have provoked some clearing of the sinus which, in this particular expert's personal experience, can be a very painful business. In Chinese medicine terms we often describe this as the energy of the area being reinstated, but because we treat the person, not simply the condition or pain which they have, there can often be secondary consequences as the system as a whole rebalances. Pathogens are often seen by the Chinese are 'going into' the system, so the process of health can lead to a reverse of this which can cause ripples in the energy as they leave. This can leave the patient feeling a little unwell for a day or two.
It is very important to let the practitioner know exactly what has happened. There may be aspects of the treatment which he or she can adjust. Some people are highly sensitive to treatment, and using fewer needles with less manipulation can make a big difference to the experience of the treatment without lessening the effect.
At any rate we hope that you feel comfortable with continuing, and hope that the treatment deals with your pain