After 21 days of no bleeding, if the bleeding which starts meets the criteria of haydh, then it should be taken as haydh. Criteria of haydh is bleeding should be for a minimum of three days and for the three days, bleeding should have continuity. If the bleeding goes over 10 days or there is a gap of less than 10 days between two separate bleedings , then it will be istihadha
In regards to going to the zarih, it makes no difference whether you use a tampon or pad. It is allowed. If your bleeding is haydh, then it is a problem going to the zarih whether you use a tampon or not.
During travel, usually water is available unless you feel that the water would be detrimental to your health for example if it's winter and is cold or similar problems. In this case you can do tayammum instead of ghusl.