Two possibilities. Either one the pregnancy is going to be fine and it's just too early to detect it with that particular ultrasound. In that case, the heartbeat should definitely be detectable in a few weeks. Two there might be something wrong with the pregnancy (either a miscarriage or ectopic).
Not good news. We should be able to detect the cardiac activity of the fetus by 5-6 weeks after a missed period with modern ultrasound equipment. No heart tones are the definition of pregnancy loss if they were previously there - or just a normal state before 5-6 weeks.
Depends on age. That depends, sometimes the fetus just isn't in a good position to hear the heartbeat. Sometimes the heartbeat can be detected as early as 9-10 weeks, but other times it can be normal not to hear it until 12- 14 weeks. If it was heard for a while, then your doctor can't hear it, they will usually send you for an ultrasound to take a look at the baby's position.
You might think so. You might think so if you compare us to hummingbirds. But I'm not aware of any research in humans to suggest that that is the case, assuming the rapid hr is still in the normal range. (60-100 at rest).
Resting Tachycardia. High heart rate at rest usually is due to some problem like fever, lack of oxygen (hypoxia), arrhythmia and few endocrinological disorder like hyperthyroidism, pheochromocytoma. It can also due to some medication if you are taking them.
Poor indicator. Pulse rate is a poor indicator to sex of child. You have a 50/50 chance of having a girl.
No. No real correlation between heartbeat and baby sex. Pregnancy myth says that a heartbeat over 140 will be a girl. Remember no scientific evidence for above.
Myths. There are many circulating myths about predicting fetal gender. String tests, heart rates, position, etc are all myths. They likely persist because they are right half the time. People selectively forget how often these myths are proven wrong, but that is part of how silly these are.
What you mean. By at the same time? Low follow by fast? Any other symptoms? A better history and physical exam needed by your physician. Possible causes caffeine, energy drinks, dehydration, hyperthyroid state, anxiety/panic attacks or arrhythmias. Get an ekg.