I inserted half a tablet of vitamin c (250mg) in my vagina after intercourse in attempt to counteract the unprotected sex. The other half, after 8 hours since the last. Several hours later I started to bleed a little. It was red and had bits of strings. And lasted for a couple of hours. It is fading now, but, is this something I should worry about?

What on earth did you think this would do?
Vit C is NOT to be used as a method of birth control, EVER!!

No one has proven that Vitamin C works as a morning after pill (many fertilized eggs abort spontaneously anyway, but numbers aren't known, although considered to be high). It's usually ingested in large doses, not inserted in the vagina. Women do claim that it helps prevent (and even cure) vaginosis and other conditions that cause odor, when inserted into the vagina in suppository form.
Your stomach lining is a Wonder of the World. It can take tough muscle meats and tough cellulose vegetable fiber and turn it to mush using powerful acids, as you know if you have ever vomited. Your vagina is soft and delicate tissue with tons of blood vessels right under the surface. You could have bled from irritation from a concentrated pill (vs a watery solution of acidic products), or you could have had some menstrual tissue, or both. I wouldn't worry about it.
PS: You can still be pregnant...

The vitamin C is a myth and inserting it can cause damage, there are professional medication for this. This is one of the silliest things to do I have heard in a while.

You can still be pregnant.

No one has proven that Vitamin C works as a morning after pill (many fertilized eggs abort spontaneously anyway, but numbers aren't known, although considered to be high). It's usually ingested in large doses, not inserted in the vagina. Women do claim that it helps prevent (and even cure) vaginosis and other conditions that cause odor, when inserted into the vagina in suppository form.
Your stomach lining is a Wonder of the World. It can take tough muscle meats and tough cellulose vegetable fiber and turn it to mush using powerful acids, as you know if you have ever vomited. Your vagina is soft and delicate tissue with tons of blood vessels right under the surface. You could have bled from irritation from a concentrated pill (vs a watery solution of acidic products), or you could have had some menstrual tissue, or both. I wouldn't worry about it.
PS: You can still be pregnant...

Yes.I am aware. Although, it gave me a bit of relief about maybe not having some permanent damages to my body. That was stupid of me to do that. I won't ever try risking it again. Thanks for the reply.