If you are sore for more then 3-4 days after your workout, that is a red flag that something is very wrong with your diet and training. You should be sore for a day, maybe two after your workout. Being sore 3+ days after your workout is a red flag that your body is not properly recovering from your workouts.
You could very easily be over training but I suspect that you most likely aren't eating enough protein. Eating a high protein diet is very important to ensure that your body recovers optimally from your workouts. Eat a minimum of 1g of protein per pound of bodyweight each day.
Whole food is always your best option but if you aren't a big eater, you can use a high quality whey isolate supplement to help meet your daily protein requirement. Most protein powders at a local supplement shop like GNC are absolute garbage. All protein powders are not created equal. You really need to be a smart consumer in today's market and do your research. Look at the ingredient list and supplements fact panel on every supplement before buying it.
For a whey protein - you want to see nothing but that in the ingredients list. Whey protein, natural flavours, a natural calorie free sweetener like stevia and then maybe some salt, soy lechitin and cocoa powder. There are a lot of good products on the market. I personally use isolation from n2bm.com. A good whey isolate should be pretty much fat and carb free. Isolation has 26g protein, 1g carb, 0g fat per 30g scoop.