Hi,

These high needs babies are very hard to cope with and put a lot of strain on relationships. It is important you work as a team. Working out what is happening and how to help requires some detective work - sometimes difficult when you are sleep deprived.
Working out what is going on is like throwing a 1000 piece jigsaw puzzle onto the floor and trying to put it back together - without ever having seen the full picture.
Firstly, how many times would you say she feeds in 24hours? How is she growing (just roughly - you would have expected her growth rate to slow down after 12 weeks)
Are there any allergies or food intolerances in either side of your family? Eczema? Asthma?
What is her sleep pattern like? How much does she sleep in 24hours?

Probably, first I would book in to see a paediatrician - just make sure everything is OK.
No mother should leave hospital without a sling. Often this behaviour is the result of overtiredness, and the need to be close to mum. Babies will often sleep really well in a sling (reflux babies often like an upright one - like the ABA simplicity sling) Sometimes babies may resist it at first - persist! And if they are worn correctly you won't feel the weight of your baby. Often, the more a baby sleeps, the more it will want to sleep. Even at this age a baby will need to sleep every 2 hours. Has some-one explained the "tired signs" to you? This could include whinging(!) Sucking fists, even when you know they have had a feed and must be full, being engaged, playing and smiling and then suddenly "cracking it". Rubbing eyes etc. At this point a sling can help get them to sleep. You will often find that the more they sleep the more they want to sleep.
You might look in your extended family for signs of allergies if there is none in yourself and your partner. Breastfed babies often don't display many obvious physical symptoms of allergy - eg eczema or asthma - breastfeeding is protecting them at this stage, but their immune system is working overtime to keep them together.
Anyway - enough for now!
Let me know how it goes