How often do tubal pregnancies happen?

i found out this ped tuesday that im pregnant after ttc for over a year (yay!) i found out at the hospital because i was having bad dizzy spells. well the doctor said he was a little concerned about how bad the dizziness is and that my blood hormone level was so low that it barely detected the pregnancy but the urine test they ran showed up well. he said that because of this he's a little worried that it might be a tubal pregnancy (even though he didnt give me an ultrasound) i know i would be somewhere from 3-5 weeks pregnant (it's hard to pinpoint because my cycle was very irregular. im just wondering how often do tubal pregnancies happen and what are signs i should look for? im so terrified about this and im angry the doctor didnt give me an ultrasound. any help will be very appriciated. thanks!!!!