Help on being stronk

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So I started doing a a lot of running and cut out soda completely this summer and I've lost almost 15 pounds already (was 160, now about 145) and I am now slightly below average in fat which is great but thing is I am on the very low end of weight for a 6'1 male because I lack a lot of muscles in my upper body mainly so I thought I'd start eating more protein in my meals etc. and start doing lifting with my running. My main question is, when to work out after eating? Do I work out immediatly after the meal because I tend to cramp if I do or can I wait an hour or so? I tend to like to work out in the afternoon so will I still get the nutrition to build my muscles from breakfast/lunch? Thanks!
pic is in inches how pathetic my forearm width is.