I'll be visiting Myanmar during February and March and was wondering how necessary is for me to take Malaria pills (I heard about nasty side effects).
I will not go camping or sleeping outside but will visit country side and go trekking.
The nasty side effects are more likely from mefloquine than from atovaquone/proguanil or doxycycline. Given "trekking" you should definitely take prophylaxis - and be sure to start the course before you arrive in Myanmar.
I (no doctor!) would advise against mefloquine, though that is cheaper. The side effects can be nasty (in my case vision quite badly affected for a month or so, and possibly mildly for years thereafter). This was not from the first couple of times I took Lariam (tm) (no longer even sold in USA) and in case there is any build-up effect it may be best to save it for where it is the only recommended drug.
If travelling to high risk malarious areas, remote from medical facilities, carrying emergency malaria standby treatment may be considered.
CDC advice (2017) regarding malaria in Myanmar is, as well as check with your doctor:
Areas with malaria: Present at altitudes <1,000 m (3,281 ft), including Bagan. Rare transmission above 1,000 m.
Estimated relative risk of malaria for US travelers: Moderate.
Drug resistance4: Chloroquine and mefloquine.
Malaria species: P. falciparum 60%, P. vivax 35%, P. malariae, P. ovale, and P. knowlesi rare.
Recommended chemoprophylaxis: In the provinces of Bago, Kachin, Kayah, Kayin, Shan, and Tanintharyi below 1,000m (3,281 ft): Atovaquone-proguanil or doxycycline. All other areas with malaria: Atovaquone-proguanil, doxycycline, or mefloquine.
Note 4: Refers to P. falciparum malaria unless otherwise noted.