Your armory:
http://eu.wowarmory.com/character-sheet.xml?r=Ghostlands&n=Discus ?
Your guildies are wrong; your gear is fine. Bosses hit hard on heroic, that's just something your healer has to get used to. Block-value is good, but using a trinket-slot on it just isn't worth it. Drop the blockvalue-trinket to get some stamina (got Essence of Gossamer, Commendation of Kael'Thas?) or anything else, really :)
Which nexus-boss was it? The frozen commander-guy has a whirlwind that hurts, Ormorok throws out ice spikes for 10k damage. Anomalus hits like a truck (+ magic damage). Telestra is just a pain. Keristrasza... Keep that bubble-wall off cooldown. This entire instance is a dirthole full of burst-damage to counter.
Edit: Apparently I skipped page 2 when reading through this >.>
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Kieras <- armory
Proud cook of
Savory Deviate Delight