Can someone be laid off while on disability?

Not necessarily.  There are several laws that protect pregnant woman from being terminated from their job.  One is The Family and Medical Leave Act ("FMLA").  Unfortunately, this law will not apply in your case since, among other requirements, the employer must employ 50 people or more. 

However, The Pregnancy Discrimination Act (PDA) provides that women affected by pregnancy, childbirth or related medical conditions shall be treated the same for all employment-related purposes, including receipt of benefits under fringe benefit programs, as other persons not so affected but similar in their inability to work.  Therefore it is unlawful sex discrimination for an employer to have leave of absence policies that treat disabilities due to pregnancy, childbirth or related conditions less beneficially than disabilities due to other medical conditions.  This applies to business employing 15 or more employees. 

Also, the Pregnancy Disability Leave Law (PDLL) requires employers who employ 5 or more employees to provide pregnant employees who are disabled by their pregnancy up to 4 months of leave.  Furthermore, under the PDLL an employer may be required to transfer a pregnant employee to a different job position.  Additionally, following a leave taken under the PDLL, a pregnant employee who is qualified to take leave under the California Family Rights Act (CFRA) may also have the right to take an additional 12-week leave.  Therefore, in the right circumstances, a pregnant employee may be entitled to 4 months leave under the PDLL, plus an additional 12 weeks of leave under the CFRA.  For most purposes, employees who are on pregnancy disability leave must be treated the same as employees on other types of disability leave in terms of pay, benefits and other terms and conditions of employment. 

It is unclear, what if any leave you applied for.  Also, an employer can lay-off a pregnant worker as long as that is not the reason for the lay-off.  So if there is a general lay-off of several employees or your job is being eliminated, you can be discharged.  However, the fact that someone else was hired to take-over your position strengthens your claim. 

At this point you should speak with an employment law attorney.  Most consultations of this type are free.