Pain when being penetrated deeply?

Im a straight young female and In certain positions where I'm being penetrated quite deeply it can hurt inside,,,and even ache for a day or so after. is there a possible medical reason for this and is it worth asking a doctor or it it normal? i heard something about if you have a "short" vagina or if they have a large penis then it can be because they're hitting your uterus wall

They can't be hitting your uterus wall, but they might be biffing your cervix.

As well as talking about it to a doctor, the other obvious thing to do is not go for deep penetration positions where you're not the one in charge of it.

How aroused are you when the sex happens?

There are a fair few reasons. I'd suggest you go see a doc about it.
Some of the possibilities I know about are and abnormal uterus (short or tilted), endometriosis (womb lining outside of the womb), IBD (in this case it would be the pressue it puts on your bowels together with having IBD and you would probably have noticable bowel issues other tiems too), issues involving pressure on your bowels or bladder (there are a fe reasons you could have a problem with that), polcistic ovary syndrome (cysts on your ovaries). The list goes on.

If you go to a doc thy will probably give you some tests like ultrasounds and internal exams to see what the cause is. There are so many potential reasons that going to the doc is the only option. Most reasons aren't too serious, but a few can become worse over time or affect fertility.

Basically asides from possibly your first few times sex should not hurt and you should see a doc if it does.

Do you have normal periods? If you have any abnormalities involving periods, bowels, bladder, stomach or generally anywhere else? If so you should mention those too.

thank you! if this kind of pain is not normal i do want to go to the doctor... just a bit embarassing. coincidently i do think I have IBS at least to some degree and was going to go to ask them about that anyway... so I might mention this as a side thing although I dont see why it would be that as it only hurts it doesn't make me need to go to the toilet.

I still suspect this is your cervix being biffed - it's a 'bruised' sort of pain, rather than a jabbing one, yes? - but as mentioned, talking to a doctor is always a good idea.

yes it's like a bruised pain not a stabbing one... like discomfort that can turn into pain if it continues. not unbearable unless sometimes if it goes on too long then my lower abdomen can ache the next day. if it's my cervix then is that normal?

It sounds like the usual response to having your cervix biffed, yes.