Question: If I use tampons during my period, can I still consider my first 5 days infertile?
Answer: Remember that it's not always all 5 days at the beginning of your cycle that are infertile. The days of Heavy and Moderate flow, when you know that you are having a true menstruation, are infertile. (In a true menstruation, the bleeding appears normal to you, and you saw all your normal signs of ovulation 9-16 days before it.) When you have days of Light or Very Light bleeding, you check for cervical mucus as usual, and when there isn't any, that day is infertile.
Tampons prevent you from checking for cervical mucus. If you're using them on days of Heavy and Moderate bleeding of a true menstruation, you should be fine to consider those days infertile. If you're using them on Light or Very Light days, I'd advise you to consider those "days of doubt," since you don't know whether cervical mucus has begun appearing. You'd call those days fertile and follow them with a Count of 3 more fertile days. If you're charting for birth control and hoping for the maximum number of infertile days in a cycle, it's nice to use pads or pantiliners on days of Light or Very Light bleeding.