Your memory is fine in terms of php but your host needs to tweak your mysql configuration(my.cnf) if is maxing out so low and then restart mysql.
I don't think any shared host could give you unlimited memory though many offer free bandwith and storage.
Your host needs to tweak these in my.cnf
sort_buffer_size
read_buffer_size
read_rnd_buffer_size
join_buffer_size
max_connections
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