Mango sapling dying

Question

So i've been growing this mango sapling for like 4 months. At first i used the toothpick method to get the seed to sprout and after it got it's first leaves I transplanted it in this pot. It all seemed to be ok until the leaves started to get yellowish and some dark spots apeared on them and the new growth would fall. And allso sometimes there appears to be some kind of spiderweb on the leaves but I haven't seen any spiders on them. I kept it in the sun and gave it water every 2 or 3 days. I would really apreciate if anyone could tell me what is wrong with my plant. Thanks!