Something Iam Not Understanding. .. Is All The Time My

Something iam not understanding. .. is all the time my peiord comes on the 18 of each month. But sense I been fixed three years ago it will come one day be gone the next. Then be back like 4 days later an be gone again? I have a app called pink pad I keep track of it. Then last month it was one day then two weeks later it came two days then nothing till this month . Some are very heavy others are lights at times