Yes, wear pantyliners - you dont want it going in your underwear.
I am not sure if it means your period is coming, especially as you have had this for 2 years, but it could be your hormones and puberty, which may trigger your period.
You need to see what colour your discharge is - thats important, then look it up - even search on google. Cos different discharge means different things, i.e infection, STI etc or just normal.
How do you tell your Mum????? There is nothing to tell at this point! It depends on how close you are with her too. If you are concerned about your discharge, talk to her. Go talk to a doctor.
She won't act any other way apart from supportive. Believe me, she had to go through all this too.
And prob still does!