I have been taking 100mg of losartan for about a year. On 4/10 I also started taking 25mg of hydrochlorothiazide. I seem to be tired all day now and my blood pressure drops to 86/66 within 30 minutes of taking the medicine and progressively increases through the day. This afternoon I started having mild chest pains on both sides. Is this something I should be worried about?

Could be. Here is the patient information sheet on Losartan.
Patient Information (http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000958/)

Please read section on side effects