I Have Been Working Out 3 Days A Week With

I have been working out 3 days a week with a trainer since last April. We rotate working different parts of the body and do circuit training every 6 or 7th session. Only during circuits do I get lightheaded and feel faint. Tonight was the first time I had to stop and cut my session short because I felt more light headed than usual and I got a sharp pain in my left temple. I felt out of balance and had to sit quite awhile before going home. I am on half of a 25mg atenolol once a day for sinus tachycardia and have been for 2.5 years. What would cause the sharp pain in my temple?