Why did i have such a bad trip?

so i used to smoke pot, and it was all good, fun trips and happy. i stopped smoking for about a year. but did ectacy 3 times. then the other day my house was empty and i was felling a little bummed out, nothing major, so i decided to swipe some 'weed' from my older brothers room. he is a real big pot head and i figured it would be safe. i only had four little hits. i didnt want to be really high, just a little. so it kicked in and i got scared. i was really hot and my heartbeat was very loud and fast. i was starting to freak, thinking i was going to die. i could barely feel anything when i walked. i have never considered caling for help until that moment. but i just waited it out and managed to calm myself down. the rest of the trip wasnt as bad but it wasnt great. so does anyone know what was up with that bad trip? seriously thought i was going to die. maby the weed was laced with something..idk but it wasnt fun. and now two days later i feel sad and very unmotivated for no reason.