You need to chill chick, have faith in your body to look after and grow this baby.
I slipped over in bathroom at 20 weeks and was so so worried I had hurt myself and the baby or ruptured something, I could not stop crying and didn't stop until I felt movement but all was well and has remained well Now that scared me but I did not call the doctor or reach for the Doppler, I had to trust my body and monitor myself for pain/bleeding.
I never wanted to get a Doppler, I have access to them at work and I have only used it once for fun so my colleagues could hear the HB whooshing away and even then the nurse picked up mine before the baby. At 18 weeks you should start to begin to feel flutters/bubbles but you do not need to panic if you do not feel everyday or very much at all, the baby is still free floating and may not always be able to touch the sides of your uterus It can be down to your body size/shape, position of Placenta as my friend found out who is 2 weeks ahead me and yet I was feeling way more before her. I am 22 weeks and I feel movement all the time, sometimes crazy activity where I just want to chuckle to myself when I feel it, sometimes dad can feel it sometime not, it depends. A lot of the movement comes when I am sitting, eating dinner, chilling. I also read that there are no receptors on the inside wall of the uterus what you feel is other tissue receptors responding like abdominal muscle I guess. (Dr Fluffy may correct me here?)
You can try and encourage movement with music at this stage, that's how I got the baby moving in the 18 weeks period and by 19 weeks I did not need the music anymore, I could just feel bubbles and as the days and weeks pass the movements get stronger and they will amuse you
I just looked up in my pregnancy app that your baby is probably measuring about 14.5-15cm which is size of small ruler and weighing probably about 210-240g's now. Do you have an app on your phone, I love mine its simply 'Pregnancy App' from Health and parenting avail on iTunes. I have books as well but I like to look and read this to keep up to date as I know that I do not expect to see my baby again until we meet face to face.
Relax and enjoy your pregnancy X