Hi. I'm A Female Age 59 And Taking A New

Hi. I'm a female age 59 and taking a new medicine synthroid for thyroid at 75 mg. I'm having constant bowel movements not loose, but like pasty and sometimes in small amounts, they appear to be dark, not normal brown, is this common? The pill is a blue color. I had a colonscopy one year ago and it was normal, no findings. Is it possible that I'm not drinking enough fluids? I take it in the morning and wait 30 minutes after before drinking coffee or eating. I also take a multi vitamin one a day. and simvastatin 20 mgs. Thank you