What Causes Abnormal Periods Inspite Of Having Menstrual Irregularity?

HI, I m an 18 year old girl that s kind of concerned. last night I started my period after not having it for three months straight (I have been sexually active, but I haven t had any symptoms of pregnancy). I m used to having irregular periods because i m only about 98 pounds, but this time it s not like a regular period. the consistency is less like liquid blood like a normal period and more like thick mucus. it s a normal colour and everything, it s just concerning that it s so thick after not having it for so long. i m also not on birth control or any other medications. thank you for your time