Your seniority date is established 31 working days after you are hired. That is for bidding purposes and vacations. If you have achieved that, then no worries.
National Master Art 16 Sec 2 applies, "Inability to work because of proven sickness or injury shall not result in the loss of seniority rights". There are some limitations, contacting your Business Representative would be prudent if you will be off an extended period. Get a withdrawal card if you don't work in a month or you will build a dues delinquency.
Merry Christmas and I hope your time off is short.