Because 2018 tax returns can only be mailed and the IRS has stopped processing all mailed returns due to the coronevirus it is likely to be a long wait for 2018 to be processed. You can still file your 2019 tax return and use 0 (zero) for your 2018 AGI.
I don't know about the Stimulus Checks but have a question for you. How did you just file 2018? Did you buy and install the 2018 Desktop program and mail in the 2018 return? Because if you used the Online version and efiled it is only for 2019 returns. So you may have filed your 2018 as a 2019 return.
Yes you should.