How to remain calm?

Ever since my best friend killed himself, I’ve been having issues with remaining calm when I go longer than expected without hearing from people I care about – especially if I talk to those people on a regular basis (generally at least once a day). So if I go two or more days and I don’t hear from them, or can’t get a hold of them, I automatically get sick with worry that something bad happened and can’t focus on anything else. I start to feel sick to my stomach, I get major stomach pain, headaches, my heart races, etc. Are these signs of PTSD?

What can I do to convince myself that things are probably okay? How do I remain calm? :(