hydrochlorothiazide-lisinopril and exercise in triathaletes

I have been on a beta blocker (labatolol 100 mg) and 50mg hydrochlorothiazide for over 15 years. I got a second opinion from another doctor mentioning my concerns in relationship to exercise. He prescribed 40 mg lisinopril and continuing the hydroclhorothiazide.After 1 week my resting blood pressure is better conrolled but after strenuous exercise my bp dropped to 80 over 50 and my ability to perform exercise crashed. I have a call into my doctor. What do you think is happening? Hopefully it's something that can be remedied with a change in dosage. Maybe my body just needs time to adjust. 80 over 50 scares me. Prior to exercice it's about 125 over 80.