Weird Behavioral Change In My 5 Yr Old

Dear moms,

My son (5 yrs) has always been a good child. He was pampered with love and gifts by all at home, since he is the first grand-kid/nephew so everyone loved him so much.
He is usually a very humble kid, very loving and affectionate. He gels with everyone around him.
A good, caring brother too, to his 2.5 yr old sister. Both share a great love.
He has always been appreciated by his teachers for his well-manners and discipline.

However, since recently he has changed to a very stubborn, adamant and cranky kid. He can't be controlled by words or any smooth disciplining. He throws tantrums, shouts, yell and sometimes hits me/others at home.
The frequency of such tantrums are more, and he does that at least every day. Sometimes 2-3 times a day too.
He shouts, screams, throws things, refuse to stop/listen to us, cries loudly and is very aggressive. The tantrum sessions last for 15 mins. But the effect is so high, that I will get an head-ache, and can feel my BP rising with that.

The tantrums are for minor things like "broken toy"," unwilling to share the toy with sister", "demanding anything" etc

He, however remains as a coolest student at School, and obviously shows different /well manners when his dad is around.

My H used to hit the kids if they are too naughty/uncontrollable. But I am against to it, and try to calm them down by other means.
But I feel guilt that I had to take the cane last night when he was too much uncontrollable. Although I did not hit, I can't guarantee myself the next time.

There are 3 things/changes that happened in his life. May be, I guess they have influenced him.

1) He started going to School.
New School, new environment, new teacher. He liked it initially. But lately complains that he doesn't have friends, over loaded with studies, and strict teacher.
He often find excuses not to go to School in the morning. He says he misses home, he needs rest, he wants to play with his sister etc...

2) I have become sick, and was resting
I have become sick since Dec 15. Due to high dose of medication, I was often felt drowsy, and needed some rest. I did not have much break from the office, so it wasn't easy for me to handle both home and office with this sickness.
During Feb-Mar, I was too sick and had frequent hospital trips. Due to it, everyone was down, and emotionally it was a big roller coaster. So, I wasn't unable to spend quality time with my kids. I hardly played with them. I would shout at them if they were making loud noise. I didn't have the patience to adjust with their childish nature; thus I would lose my temper often.
Most of these days it was my mom and hubby who took care of the kids, like feeding, bathing, dressing, teaching etc.. So, obviously they missed me.
Now, I am fine, recovering and spending time with the kids as before.

3) During my sickness, I was little easy on kids too. I allowed them to watch too much cartoons without screening them. One of them were "spectaculars spider man cartoon". It seemed violent, and I worry my son has learnt to hit others from it.

Is it normal?

As a remedy, I have stopped reacting to his tantrums. I neither try to control him, nor giving in to his demands. I just ignore and pretend as if I haven't heard him. But I coolly attend to him the next minute when he stops. I just don't show my temper, but lovingly converse. So, his tantrums doesn't last long, as well he doesn't hit anyone.
No tantrums for the past 2 days though...

Please help me