Can i be on my period and still get pregnant

Original question:
I had unprotected sex and he ejaculated in me, and four days later I get my period. Could I still get pregnant since sperm lives in me for 5 days, and it’s just the 4th day?
Okay it’s time to kill two birds with one stone. I’ll stick with answering this question and won’t mention STDs (someone else has done that already).
I don't know your age but whatever it is I know the “Am I pregnant?” question is very distressing so I’ll answer this in a way that should calm you down in future cases too.
SEX EDUCATION
The menstrual cycle
Now, since you’re menstruating by now you at least know that it happens periodically (period is periodical, lol why am I now realising this word play. Don’t mind me)
So medical literature will tell you that an ‘average’ period will last 28 days (plus or minus 7 days) and this helps them explain to you what happens.
Let me put a picture of the cycle here to help me (bear with me though, this part is important)
Now, I wont delve too much into detail here (since this website will do it for me :Stages of Menstrual Cycle: Menstruation, Ovulation, Hormones, More
), but what you should know is that :
the start of menses is the first day of the cycle
the cycle varies, more in some than others
the part that varies is usually the first half of the cycle i.e. the first 14 days of a 28 day cycle or first 15 of a 30 day cycle.
The second half of the cycle is PRETTY close to 14 days all the time
The egg is released 14 days before your menses. (mid-cycle if the whole cycle was 28 days)
The egg lives for 1–2 days
So from what I’ve listed here, the egg dies off about 10 days (max) before you first see blood. If your cycle is shorter this changes but whatever the case the egg comes out in the middle and lives for 1–2 days.
Got it so far?
I don’t know the details of your cycle so you need to calculate this. There are pretty good apps to track your cycle approximately. Once you are sexually active, it’s good to get one help you.
2. The sperm
So now you’ve gotten the right idea about sperm living long but its really closer to 3 days than 5.
Still lets just say we are speaking of the worst case scenario and they lived that long, the sperm can only fertilise an egg which isn't dead. Remember that I mentioned before that the egg comes out 14 DAYS BEFORE THE MENSES? Well the sperm need to meet it in that period of time when its alive. If it really was your menses then the egg had approximately 10 days to die.
3. ‘IF IT REALLY WAS YOUR MENSES’
This doesn't count if it was Abnormal uterine bleeding like for example inter-menstrual bleeding or implantation bleed. Here’s another site: Abnormal (Dysfunctional) Uterine Bleeding
Or it could be bleeding from implantation (it feels a bit different from your usual period)
4. THE ONLY SURE WAY TO KNOW
is a pregnancy test looking for BHCG in the urine (those strips you pee on)
5. NEXT TIME…
Monitor you cycle properly and you would be able to tell where in it you are so you can easily answer this question without panic
TLDR
If it is really your period that came 4 days later, the egg is most likely dead.
If your cycles are regular then this answer is a bit more certain
Can you still become pregnant? Yes
Why? because sometimes it is not your period that comes and some people don't know or cant tell the difference and hence they might be right smack in the middle of their cycle (or pregnant already)
And uh yeah that's it. As always visit a gynaecologist when in doubt (and to regularly check for STDs) and you should be fine.