Anyone else take methadone?

I've been on methadone for about a year, but only taking about 15 to 20mg a day. so not very high doses, but for some reason lately i've been getting spells, that are like 15 minutes of fast pounding heartbeat, really lightheaded, and shaky or trembly feeling afterwards, and feelings of impending death. has this happened to anyone else? someone else told me that it was anxiety? thanx