Short version: the body needs proteins to function and repair. If you do not provide adequate amount of proteins in daily diet - day after day - it will first use stored blood proteins and then start breaking muscles. Since most likely you are MO - you also may have a lot of muscle s- but even as you lose weight - you want to lose fat and not muscles.
Less muscles - lower BMR = possible lower weight loss.
add also so many other issues: slow healing wounds, hair loss, skin issue, vision issue, muscle pain and cramps (while the body is breaking down and consuming its own muscles) and so on.
First 1-2 months - you most likely have some reserves.. but now - you need to step it up and get as much as you need it.
There are ways to get proteins. Shakes are one - but there is teh Greek yogurt, cottage cheese, cheese, fish, meat, eggs... egg whites... and so on....
when in doubt - google it
http://www.google.com/#hl=en&gs_nf=1&cp=20&gs_id=26&xhr=t&q= protein+deficiencies&pf=p&sclient=psy-ab&oq=protein+deficien cies&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=fe6f8 da2d5e72e38&biw=1323&bih=897