I think the Huo family killed him. Although Liu Fuling has the support of Huo Guang, although Huo Guang actually holds the power in the DPRK, he is a courtier after all, and can't give orders to the world at will, and his political views don't conform to Liu Fuling's, so Liu Fuling won't listen to him in some things. This is because he had to kill Liu Fuling to realize his political ideal. From his support for Liu He, we can see his ambition, because Liu He in history was dissolute before he ascended the throne. If he really wanted to be a good emperor for the people all over the world, he would definitely not support Liu He. Because there is only one puppet emperor, he can be the puppet master behind him. As for him, Liu He was later abolished because Liu He didn't know how to please Huo Guang when he became emperor, but he took everyone around him when he was king of Changyi.
Historically, Liu Fuling was very strong. He fell ill and died suddenly at the age of 19 and 20. This may be because someone poisoned him for a long time, and the toxin accumulated to a certain extent and suddenly broke out.
And the Shangguan queen's surname is Shangguan, not Huo.
As for being in his mother's belly for 14 months, I think it may be a rumor from later generations or that historians want to give the son of heaven a vision when he was born (the ancients liked it). It may also be that his mother, Yi, wanted to make her children different from ordinary people, lied about being pregnant to prolong her time, and wanted to tell others that this was the phase of Fuxi and so on. Because Emperor Wu of the Han Dynasty likes to believe these things.