I Have Been Having Loose Stools, Please Advise Me

I have been having loose stools for nearly one month. Haven`t changed my diet, haven`t lost weight and still have a good appetite. I was diagnosed with underactive thyroid nearly 3 years ago but haven`t had any bowel problems like this before. Most mornings I have nothing but chronic gas without any movement of the bowel but then 2 days later it feels as it everything is just falling out and I have to go 2 or 3 times in succession and the stools are always loose and I can`t control it. I am terrified this could be a sign of something very serious. I am female aged 54. Thank you