Hi, when you say you got a faint positive, did you follow box directions exactly? A second line can appear if it sits too long ( an evaporation line). I'd take another test to confirm.
No, a tampon will not cause a miscarriage. Some women spot in early pregnancy. Or it could be the beginning of a miscarriage/chemical pregnancy. How late are you for your period?
I'd take another test today or tomorrow --- let me know the result.