Hello. I would like to help you find an answer.
It sounds like one of the things you described.
Any chance of pregnancy or STD?
I suspect that this was an early pregnancy that was a miscarriage. It sounds as if this is the case.
I would recommend that you get seen by your doctor though as you need a pelvic exam to make sure that all of the products were expelled. Retained products can cause infection.
If you have follow up questions, please ask prior to accepting.
If it is an STD, likely trichomoniasis or gonorrhea just based on symptoms.
Mucous plug is possible but unlikely if you have never had one before and there is no good reason for one now.
Cramps and bleeding go along with miscarriage.
Yes, if it was an STD it would continue
Something very very slimy and sticky just came out of my vagina. I had a tiny amount of 'pinkness' with it and the thing itself is clear/beige/brown with a harder part. It is only half a centimetre long but very slimy and therefore spreads out. I have never had anything like this before. I wondered if it was either a very early miscarriage, or the mucus plug (though I'm not pregnant...or didn't think I was!!), or something infection-related. Any help would be much appreciated, I'm feeling very strange about it.