Maybe TMI-cramping and discharge

Curious if anyone has been experiencing this. I am 18weeks and every once in awhile I get a terrible cramping feeling for about 10 seconds or so and then I pass a thick amount of clear stretchy kinda milky discharge. I've been rather crampy my whole pregnancy with terrible back pain but when this hits it's rather strong and kinda makes me stop in my tracks-I'm worried this could be a sign of something bad. I have an appointment Monday and plan on talking to my doctor about it but was looking for some advice maybe. Thank you.