What Causes Fluctuation In BP Levels While Being Treated For Pneumonia?

57 y/o male with HX of multiple sclerosis dx in 1987 in reasonable health for mS....bedridden, but able to feed himself and oral. Treated for pneumonia and severe dysphasia . Had peg placement . Talkative, alert after surgery on Thursday 1/7/15. On Saturday 1/10/15. Complained of not feeling good..couldn t be specific, other than not feeling good. Went to sleep around 1pm on 1/10/15 awoke confused. Later Bp dropped. On Sunday, cannot talk, goes into deep sleep. With snoring. Difficulty arousing .LBP neg for meningitis., MRI beg. except for brain atrophy due to MS, no viral or bacterial infection in blood ie spinal fluid. Not understanding fluctuation in BP and possible cause for change in status. Pt been on ambien x5 days prior to 1/10/15