Some cannabis strains are higher in THC or CBD than other strains, and they have different effects. Maybe you had one that caused some anxiety. Or maybe, you had something else, that you aren't aware of? And remember, everybody is different, so we all react to things differently. Some people don't like how cannabis makes them feel and that's ok.
Next time you smoke, maybe try a smaller amount, and hopefully it is an Indica strain (not Sativa). Indica is usually more relaxing, and better for pain and sleep problems. Sativa is usually stimulating, exciting, and good against cancer.