I Have High Blood Pressure And I've Had It For

I have high blood pressure and I've had it for at least 30 years. I am 64 now and I'd been taking Valsatran and doing ok. My doctor changed my med to Losartan 100mg since I took the first dose today and now I'm having a pain in my back like a muscle is tightening. What's up! I will not take another dose until i reach her,