Hii ,  
Welcome To HCM & Thanks For Asking Your Query Here 
Your medical history is suggestive of 
muscle strain which may be caused by Heavy Lifting , Quick Movement ,Prolonged Sitting , improper Posture and you can take over the counter 
muscle relaxant like thiocholchicoside after discussing your Doctor
Along with medicine Cold compresses , Warm baths ,Gentle Massage may help you ,Also Avoid oily and spicy foods to avoid gas , 
bloating and heart burn which may also cause similar pain in right chest to shoulder 
However Pain Under Right Arm including armpit may indicate some serious underlying disorders including Cardiac or Neurologic origin and it needs thorough medical check up and investigations to spot a confirm Diagnosis 
You Should get it evaluated by a doctor as soon as possible
-if your symptoms do not improve over 5-7 Days 
-if your Pain under right armpit moves towards the chest, than it may be related to heart Disease
-If you feel 
numbness , 
tingling in arm or shoulder or unable to use your arm completely
so i would strongly suggest you to see Your GP to rule out any underlying medical condition 
I hope it help 
Thanks & Get well Soon