I had the same thing and they told me it was anxiety attacks.
I had a heart rate of 160 and was in the hospital for four days and they did all kinds of test, even a cath. No blockages.
They put me on bata blockers and cardezam to keep my heart rate down.
I had a burning sensation across my chest and through out my entire body. I thought I was going to die.
Yes, Pepto Bismol will make stool dark brown or black!
If you happen to be taking peptobismol or something like this for a few days, it can make your stool quite dark.
MAN WHAT I WOULD DO TO BE 16 AGAIN. ENJOY IT,IT'LL FLY BY BRFORE YOU KNOW IT.YOUR PROBABLY FINE LIKE YOUR FATHER SAID,BUT TO PUT YOUR MIND AT EASE GO TO THE DOC. AND GET IT CHECKED OUT.MOST LIKELY HE OR SHE WILL SAY YOU ARE FINE THEN YOU EON'T BE DRIVING YOURSELF CRAZY ABOUT IT.SINCE I WAS 8 I HAVE DROVE MYSELF BATTY OVER ANYTHING AND EVERYTHING,I'M 36 NOW,AND YOU NAME IT AND I THINK I HAVE IT THEN FINALLY I WILL GO TO THE DOC. AND EVERYTHING IS USUALLY FINE THAT WILL PUT MY MIND AT EASE FOR A SHORT TIME TILL I THINK I HAVE SOMETHING ELSE GOOD LUCK AND ENJOY LIFE AND STOP WORRYING.
Very dark brown naturally? Yeah, I've had that pretty often. You're primary doctor can give you a very easy take home test to see if there's blood in your stool. I would imagine that black you see on the toilet paper is just food or something and the anxiety's getting the better of you, but you never know. Call your doc's office and ask if you can go by and pick up one of the tests.