Simply, everything is Taahir for you unless and until you know for sure that it is Najis. This is a rule in jurisprudence based on a narration of Ammar from Imam Jafar Al-Sadiq (AS) stating : Everything is ritually clean till you know that it is ritually unclean. (Wasaa'il al-Shia, V.2, Section 37 on Nijasah).
Your doubt in this matter does not make any difference. You are ritually clean because you are just doubting and not sure that your body became Najis.