Got some good news. Ultrasound collected earlier this week, no abnormalities found. Took the results to my GP and he stil believes it is something inconsequential, such as a burst blood vessel, no further investigation needed. I am still a bit unsure about, e.g., if it happens again, do I pursue investigation again? I suspect it would not be best to presume innocence of similar symptoms to a previously innocent occurrence. But still, perfectly normal thing which occasionally happens to folk and does not signify serious health issue, I am told.