Memory issues are common on cheaper/poor shared hosting.
Or even on good hosting, if you use tons of plugins, a large theme, or simply are edging on the limit and have a lot of visitors on your site.
Whatever the reason, looking at the memory limit and perhaps increasing it may help.
Or go over your plugins, see which ones can be removed. Or use better plugins.
Alternatively, get better hosting if you’re on a cheap host – You won’t be the first who may outgrown their package.