Actually, the researched is mixed about whether or not multiple sonograms harm the baby and/or how many sonograms a baby can take and at what gestational age. There are many, many doctors that feel that waiting longer is better. . .and a whole body of medical folks that feel that you only do a sonogram when necessary. I appreciate your predicament in that you're worried. . .but I always think that "medicine on demand" is a bad thing. I gotta tell you to though that I know personally, when I'm stressed in a situation and think that one more thing will fix it. . .I often find something else to worry about. It's our nature.
Oh--and to answer your question. . .Nope. . .not weird at all not to hear the heartbeat at this point.