my baby is 13 weeks old. he's pretty manageable in the day time. he takes pretty short naps in the day (typically ranging from 15 mins to 30 mins). of late, he gets really cranky and cries and fusses a lot during the night (aftr 8pm). any reason why?

Sounds a lot like a case of the "witching hour". Don't worry it's not scary or anything supernatural. After turning 6 weeks, between 6 and 10pm, some babies who are usually pretty chill and calm in the day, suddenly become super-cranky and cry non stop. This time period is called the witching hour. Some attribute to tiredness at the end of the day, some say it's due to baby's hypersensitivity. In the day, when not many people are at home, it's quieter and then suddenly when the sun goes down and people come home, things become hectic around the house and baby is affected by this change. There are many other reasons as well. It will not last forever though, the whole witching hour phenomenon will gradually go away as baby gets older and get more used to things. Here's an interesting article you can read about this intriguing subject: http://www.thebump.com/a/baby-fussy-at-night

Could it be that your baby is going through a growth spurt? It usually last for 2-3 days but could also go up to a week. During which your baby will feed more and would need less sleep (hence waking up more frequently at night or taking shorter naps). Other signs would include your baby appearing more restless and clingy. Growth spurts could also cause your baby to have bowel movements at night due to digestive changes. If your baby is indeed going through a growth spurt, you just need to be attentive to our baby’s cues for more milk or attention. And very importantly, take care of yourself. Drink more water and get as much rest as you could. Here’s an article on baby’s growth spurt: http://www.babycentre.co.uk/a25012757/baby-growth-spurts

your baby is overtired. 15-30 mins naps are insufficient. they need to sleep at least 1-2.5h for each nap. you need to push bedtime earlier.