If you've only been doing it for 6 weeks, and you've gained 3-5lbs of muscle. I say you are on the right track. Consistency is the key. I lift heavy when i want to get bigger. 4-6 reps per set, 4 sets per exercise. 10-15 reps, still 4 sets per exercise with a shorter rest period when i wanna trim down. Try not to do the same exercises all the time, you need to switch it up at least once every 6 weeks; shock your muscles. You'll see growth faster that way. It works for me. I used to weight 130lbs, now I weight 210lbs. It took me a while to gain the weight, but I did it without any weight gainer or protein shakes. You might think i am one of those easy gainers.. I am not. Asians don't gain muscle that easily.

 

Another small sidenote id like to point out is FORM. i see so much bad form out there its unbelievable. Use good form and don't ego-lift or you WILL get injured - it looks really impressive to do lat pull-downs with twice your bodyweight but dislocate your shoulder and your bodybuilding is over. Don't use momentum and don't throw the weights around. If you have to cheat, then lower the weight. Everybody thinks they use good form but try this, try 3 seconds up and 3 seconds down for a total of 6 seconds for each rep - count out loud "one onethousand, two onethousand, three onethousand". If you cant do reps this slowly then you are probably cheating and using momentum to help you so lower the weight and try again.

Another thing to be mindful of when doing any exercise is to mentally note what is moving and what muscles are being used. If you are doing biceps exercises then only the elbows should be moving, not the shoulders. If you are doing abs, the neck and head should not be moving. Always be thinking, what muscle is being used now! Think about the physics involved and make sure you are not fooling yourself into thinking you are lifting more than you really are.